diff --git a/lib/web_ui/build.canvaskit.yaml b/lib/web_ui/build.canvaskit.yaml
index 3eee9e132401d..8be2596d000a7 100644
--- a/lib/web_ui/build.canvaskit.yaml
+++ b/lib/web_ui/build.canvaskit.yaml
@@ -10,6 +10,7 @@ targets:
           compiler: dart2js
           dart2js_args:
             - --no-minify
+            - --disable-inlining
             - --enable-asserts
             - --enable-experiment=non-nullable
             - --no-sound-null-safety
diff --git a/lib/web_ui/build.html.yaml b/lib/web_ui/build.html.yaml
index b469539d305da..36ea478d17df4 100644
--- a/lib/web_ui/build.html.yaml
+++ b/lib/web_ui/build.html.yaml
@@ -9,6 +9,7 @@ targets:
           compiler: dart2js
           dart2js_args:
             - --no-minify
+            - --disable-inlining
             - --enable-asserts
             - --enable-experiment=non-nullable
             - --no-sound-null-safety
diff --git a/lib/web_ui/lib/src/engine.dart b/lib/web_ui/lib/src/engine.dart
index 857d6acd1b76e..e1ab01db9a45f 100644
--- a/lib/web_ui/lib/src/engine.dart
+++ b/lib/web_ui/lib/src/engine.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 library engine;
 
 import 'dart:async';
diff --git a/lib/web_ui/lib/src/engine/alarm_clock.dart b/lib/web_ui/lib/src/engine/alarm_clock.dart
index 30a3746f4abe2..ee8e6a6be96ec 100644
--- a/lib/web_ui/lib/src/engine/alarm_clock.dart
+++ b/lib/web_ui/lib/src/engine/alarm_clock.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
 /// A function that returns current system time.
@@ -24,13 +23,13 @@ class AlarmClock {
   final TimestampFunction _timestampFunction;
 
   /// The underlying timer used to schedule the callback.
-  Timer _timer;
+  Timer? _timer;
 
   /// Current target time the [callback] is scheduled for.
-  DateTime _datetime;
+  DateTime? _datetime;
 
   /// The callback called when the alarm goes off.
-  ui.VoidCallback callback;
+  late ui.VoidCallback callback;
 
   /// The time when the alarm clock will go off.
   ///
@@ -39,8 +38,8 @@ class AlarmClock {
   /// If the value is updated before an already scheduled timer goes off, the
   /// previous time will not call the [callback]. Think of the updating this
   /// value as "changing your mind" about when you want the next timer to fire.
-  DateTime get datetime => _datetime;
-  set datetime(DateTime value) {
+  DateTime? get datetime => _datetime;
+  set datetime(DateTime? value) {
     if (value == _datetime) {
       return;
     }
@@ -72,7 +71,7 @@ class AlarmClock {
     } else {
       assert(_datetime != null,
           'We can only have a timer if there is a non-null datetime');
-      if (_datetime.isAfter(value)) {
+      if (_datetime!.isAfter(value)) {
         // This is the case when the value moves the target time to an earlier
         // point. Because there is no way to reconfigure an existing timer, we
         // must cancel the old timer and schedule a new one.
@@ -89,7 +88,7 @@ class AlarmClock {
 
   void _cancelTimer() {
     if (_timer != null) {
-      _timer.cancel();
+      _timer!.cancel();
       _timer = null;
     }
   }
@@ -100,12 +99,12 @@ class AlarmClock {
     final DateTime now = _timestampFunction();
     // We use the "not before" logic instead of "is after" because we may have
     // zero difference between now and _datetime.
-    if (!now.isBefore(_datetime)) {
+    if (!now.isBefore(_datetime!)) {
       _timer = null;
       callback();
     } else {
       // The timer fired before the target date. We need to reschedule.
-      _timer = Timer(_datetime.difference(now), _timerDidFire);
+      _timer = Timer(_datetime!.difference(now), _timerDidFire);
     }
   }
 }
diff --git a/lib/web_ui/lib/src/engine/assets.dart b/lib/web_ui/lib/src/engine/assets.dart
index 50127ab133bc9..60803b0d00560 100644
--- a/lib/web_ui/lib/src/engine/assets.dart
+++ b/lib/web_ui/lib/src/engine/assets.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
 /// This class downloads assets over the network.
@@ -17,10 +16,10 @@ class AssetManager {
 
   const AssetManager({this.assetsDir = _defaultAssetsDir});
 
-  String get _baseUrl {
+  String? get _baseUrl {
     return html.window.document
         .querySelectorAll('meta')
-        .whereType<html.MetaElement>()
+        .whereType<html.MetaElement?>()
         .firstWhere((dynamic e) => e.name == 'assetBase', orElse: () => null)
         ?.content;
   }
@@ -58,7 +57,7 @@ class AssetManager {
       final ByteBuffer response = request.response;
       return response.asByteData();
     } on html.ProgressEvent catch (e) {
-      final html.EventTarget target = e.target;
+      final html.EventTarget? target = e.target;
       if (target is html.HttpRequest) {
         if (target.status == 404 && asset == 'AssetManifest.json') {
           html.window.console
diff --git a/lib/web_ui/lib/src/engine/bitmap_canvas.dart b/lib/web_ui/lib/src/engine/bitmap_canvas.dart
index 1e200df3dd16c..b594f39ed665f 100644
--- a/lib/web_ui/lib/src/engine/bitmap_canvas.dart
+++ b/lib/web_ui/lib/src/engine/bitmap_canvas.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
 /// A raw HTML canvas that is directly written to.
@@ -13,7 +12,7 @@ class BitmapCanvas extends EngineCanvas {
   /// Painting outside these bounds will result in cropping.
   ui.Rect get bounds => _bounds;
   set bounds(ui.Rect newValue) {
-    assert(newValue != null);
+    assert(newValue != null); // ignore: unnecessary_null_comparison
     _bounds = newValue;
     final int newCanvasPositionX = _bounds.left.floor() - kPaddingPixels;
     final int newCanvasPositionY = _bounds.top.floor() - kPaddingPixels;
@@ -26,7 +25,7 @@ class BitmapCanvas extends EngineCanvas {
   }
 
   ui.Rect _bounds;
-  CrossFrameCache<html.HtmlElement> _elementCache;
+  CrossFrameCache<html.HtmlElement>? _elementCache;
 
   /// The amount of padding to add around the edges of this canvas to
   /// ensure that anti-aliased arcs are not clipped.
@@ -42,7 +41,7 @@ class BitmapCanvas extends EngineCanvas {
 
   /// The last paragraph style is cached to optimize the case where the style
   /// hasn't changed.
-  ParagraphGeometricStyle _cachedLastStyle;
+  ParagraphGeometricStyle? _cachedLastStyle;
 
   /// List of extra sibling elements created for paragraphs and clipping.
   final List<html.Element> _children = <html.Element>[];
@@ -74,7 +73,7 @@ class BitmapCanvas extends EngineCanvas {
   final double _devicePixelRatio = EngineWindow.browserDevicePixelRatio;
 
   // Compensation for [_initializeViewport] snapping canvas position to 1 pixel.
-  int _canvasPositionX, _canvasPositionY;
+  int? _canvasPositionX, _canvasPositionY;
 
   // Indicates the instructions following drawImage or drawParagraph that
   // a child element was created to paint.
@@ -101,7 +100,7 @@ class BitmapCanvas extends EngineCanvas {
   /// as the [Rect.size] of the bounds fully fit within the size used to
   /// initialize this canvas.
   BitmapCanvas(this._bounds)
-      : assert(_bounds != null),
+      : assert(_bounds != null), // ignore: unnecessary_null_comparison
         _widthInBitmapPixels = _widthToPhysical(_bounds.width),
         _heightInBitmapPixels = _heightToPhysical(_bounds.height),
         _canvasPool = _CanvasPool(_widthToPhysical(_bounds.width),
@@ -113,7 +112,7 @@ class BitmapCanvas extends EngineCanvas {
     _canvasPositionX = _bounds.left.floor() - kPaddingPixels;
     _canvasPositionY = _bounds.top.floor() - kPaddingPixels;
     _updateRootElementTransform();
-    _canvasPool.allocateCanvas(rootElement);
+    _canvasPool.allocateCanvas(rootElement as html.HtmlElement);
     _setupInitialTransform();
   }
 
@@ -138,9 +137,9 @@ class BitmapCanvas extends EngineCanvas {
   void _setupInitialTransform() {
     final double canvasPositionCorrectionX = _bounds.left -
         BitmapCanvas.kPaddingPixels -
-        _canvasPositionX.toDouble();
+        _canvasPositionX!.toDouble();
     final double canvasPositionCorrectionY =
-        _bounds.top - BitmapCanvas.kPaddingPixels - _canvasPositionY.toDouble();
+        _bounds.top - BitmapCanvas.kPaddingPixels - _canvasPositionY!.toDouble();
     // This compensates for the translate on the `rootElement`.
     _canvasPool.initialTransform = ui.Offset(
       -_bounds.left + canvasPositionCorrectionX + BitmapCanvas.kPaddingPixels,
@@ -162,7 +161,7 @@ class BitmapCanvas extends EngineCanvas {
 
   // Used by picture to assess if canvas is large enough to reuse as is.
   bool doesFitBounds(ui.Rect newBounds) {
-    assert(newBounds != null);
+    assert(newBounds != null); // ignore: unnecessary_null_comparison
     return _widthInBitmapPixels >= _widthToPhysical(newBounds.width) &&
         _heightInBitmapPixels >= _heightToPhysical(newBounds.height);
   }
@@ -367,7 +366,7 @@ class BitmapCanvas extends EngineCanvas {
   html.ImageElement _reuseOrCreateImage(HtmlImage htmlImage) {
     final String cacheKey = htmlImage.imgElement.src;
     if (_elementCache != null) {
-      html.ImageElement imageElement = _elementCache.reuse(cacheKey);
+      html.ImageElement? imageElement = _elementCache!.reuse(cacheKey) as html.ImageElement?;
       if (imageElement != null) {
         return imageElement;
       }
@@ -375,7 +374,7 @@ class BitmapCanvas extends EngineCanvas {
     // Can't reuse, create new instance.
     html.ImageElement newImageElement = htmlImage.cloneImageElement();
     if (_elementCache != null) {
-      _elementCache.cache(cacheKey, newImageElement, _onEvictElement);
+      _elementCache!.cache(cacheKey, newImageElement, _onEvictElement);
     }
     return newImageElement;
   }
@@ -386,10 +385,10 @@ class BitmapCanvas extends EngineCanvas {
 
   html.HtmlElement _drawImage(
       ui.Image image, ui.Offset p, SurfacePaintData paint) {
-    final HtmlImage htmlImage = image;
-    final ui.BlendMode blendMode = paint.blendMode;
-    final EngineColorFilter colorFilter = paint.colorFilter as EngineColorFilter;
-    final ui.BlendMode colorFilterBlendMode = colorFilter?._blendMode;
+    final HtmlImage htmlImage = image as HtmlImage;
+    final ui.BlendMode? blendMode = paint.blendMode;
+    final EngineColorFilter? colorFilter = paint.colorFilter as EngineColorFilter?;
+    final ui.BlendMode? colorFilterBlendMode = colorFilter?._blendMode;
     html.HtmlElement imgElement;
     if (colorFilterBlendMode == null) {
       // No Blending, create an image by cloning original loaded image.
@@ -410,22 +409,22 @@ class BitmapCanvas extends EngineCanvas {
         case ui.BlendMode.luminosity:
         case ui.BlendMode.xor:
           imgElement = _createImageElementWithSvgFilter(image,
-              colorFilter._color, colorFilterBlendMode, paint);
+              colorFilter!._color, colorFilterBlendMode, paint);
           break;
         default:
           imgElement = _createBackgroundImageWithBlend(image,
-              colorFilter._color, colorFilterBlendMode, paint);
+              colorFilter!._color, colorFilterBlendMode, paint);
           break;
       }
     }
-    imgElement.style.mixBlendMode = _stringForBlendMode(blendMode);
+    imgElement.style.mixBlendMode = _stringForBlendMode(blendMode) ?? '';
     if (_canvasPool.isClipped) {
       // Reset width/height since they may have been previously set.
       imgElement.style
         ..removeProperty('width')
         ..removeProperty('height');
       final List<html.Element> clipElements = _clipContent(
-          _canvasPool._clipStack, imgElement, p, _canvasPool.currentTransform);
+          _canvasPool._clipStack!, imgElement, p, _canvasPool.currentTransform);
       for (html.Element clipElement in clipElements) {
         rootElement.append(clipElement);
         _children.add(clipElement);
@@ -493,7 +492,7 @@ class BitmapCanvas extends EngineCanvas {
         targetWidth *= image.width / src.width;
         targetHeight *= image.height / src.height;
       }
-      _applyTargetSize(imgElement, targetWidth, targetHeight);
+      _applyTargetSize(imgElement as html.HtmlElement, targetWidth, targetHeight);
       if (requiresClipping) {
         restore();
       }
@@ -533,7 +532,7 @@ class BitmapCanvas extends EngineCanvas {
   // For src,srcOver it only sets background-color attribute.
   // For dst,dstIn , it only sets source not background color.
   html.HtmlElement _createBackgroundImageWithBlend(HtmlImage image,
-      ui.Color filterColor, ui.BlendMode colorFilterBlendMode,
+      ui.Color? filterColor, ui.BlendMode colorFilterBlendMode,
       SurfacePaintData paint) {
     // When blending with color we can't use an image element.
     // Instead use a div element with background image, color and
@@ -561,7 +560,7 @@ class BitmapCanvas extends EngineCanvas {
         style
           ..position = 'absolute'
           ..backgroundImage = "url('${image.imgElement.src}')"
-          ..backgroundBlendMode = _stringForBlendMode(colorFilterBlendMode)
+          ..backgroundBlendMode = _stringForBlendMode(colorFilterBlendMode) ?? ''
           ..backgroundColor = colorToCssString(filterColor);
         break;
     }
@@ -570,10 +569,10 @@ class BitmapCanvas extends EngineCanvas {
 
   // Creates an image element and an svg filter to apply on the element.
   html.HtmlElement _createImageElementWithSvgFilter(HtmlImage image,
-      ui.Color filterColor, ui.BlendMode colorFilterBlendMode,
+      ui.Color? filterColor, ui.BlendMode colorFilterBlendMode,
       SurfacePaintData paint) {
     // For srcIn blendMode, we use an svg filter to apply to image element.
-    String svgFilter;
+    String? svgFilter;
     switch (colorFilterBlendMode) {
       case ui.BlendMode.srcIn:
       case ui.BlendMode.srcATop:
@@ -591,7 +590,7 @@ class BitmapCanvas extends EngineCanvas {
         break;
       case ui.BlendMode.modulate:
         // Porter duff source * destination but preserves alpha.
-        svgFilter = _modulateColorFilterToSvg(filterColor);
+        svgFilter = _modulateColorFilterToSvg(filterColor!);
         break;
       case ui.BlendMode.overlay:
         // Since overlay is the same as hard-light by swapping layers,
@@ -655,11 +654,11 @@ class BitmapCanvas extends EngineCanvas {
     double x,
     double y,
   ) {
-    html.CanvasRenderingContext2D ctx = _canvasPool.context;
+    html.CanvasRenderingContext2D? ctx = _canvasPool.context;
     x += line.left;
-    final double letterSpacing = style.letterSpacing;
+    final double? letterSpacing = style.letterSpacing;
     if (letterSpacing == null || letterSpacing == 0.0) {
-      ctx.fillText(line.displayText, x, y);
+      ctx!.fillText(line.displayText!, x, y);
     } else {
       // When letter-spacing is set, we go through a more expensive code path
       // that renders each character separately with the correct spacing
@@ -671,10 +670,10 @@ class BitmapCanvas extends EngineCanvas {
       // would put 5px before each letter and 5px after it, but on the web, we
       // put no spacing before the letter and 10px after it. This is how the DOM
       // does it.
-      final int len = line.displayText.length;
+      final int len = line.displayText!.length;
       for (int i = 0; i < len; i++) {
-        final String char = line.displayText[i];
-        ctx.fillText(char, x, y);
+        final String char = line.displayText![i];
+        ctx!.fillText(char, x, y);
         x += letterSpacing + ctx.measureText(char).width;
       }
     }
@@ -688,9 +687,9 @@ class BitmapCanvas extends EngineCanvas {
     if (paragraph._drawOnCanvas && _childOverdraw == false) {
       // !Do not move this assignment above this if clause since, accessing
       // context will generate extra <canvas> tags.
-      final List<EngineLineMetrics> lines = paragraph._measurementResult.lines;
+      final List<EngineLineMetrics> lines = paragraph._measurementResult!.lines!;
 
-      final SurfacePaintData backgroundPaint = paragraph._background?.paintData;
+      final SurfacePaintData? backgroundPaint = paragraph._background?.paintData;
       if (backgroundPaint != null) {
         final ui.Rect rect = ui.Rect.fromLTWH(
             offset.dx, offset.dy, paragraph.width, paragraph.height);
@@ -702,7 +701,7 @@ class BitmapCanvas extends EngineCanvas {
         ctx.font = style.cssFontString;
         _cachedLastStyle = style;
       }
-      _setUpPaint(paragraph._paint.paintData);
+      _setUpPaint(paragraph._paint!.paintData);
       double y = offset.dy + paragraph.alphabeticBaseline;
       final int len = lines.length;
       for (int i = 0; i < len; i++) {
@@ -718,8 +717,8 @@ class BitmapCanvas extends EngineCanvas {
         _drawParagraphElement(paragraph, offset);
     if (_canvasPool.isClipped) {
       final List<html.Element> clipElements = _clipContent(
-          _canvasPool._clipStack,
-          paragraphElement,
+          _canvasPool._clipStack!,
+          paragraphElement as html.HtmlElement,
           offset,
           _canvasPool.currentTransform);
       for (html.Element clipElement in clipElements) {
@@ -743,8 +742,8 @@ class BitmapCanvas extends EngineCanvas {
 
   /// Paints the [picture] into this canvas.
   void drawPicture(ui.Picture picture) {
-    final EnginePicture enginePicture = picture;
-    enginePicture.recordingCanvas.apply(this, bounds);
+    final EnginePicture enginePicture = picture as EnginePicture;
+    enginePicture.recordingCanvas!.apply(this, bounds);
   }
 
   /// Draws vertices on a gl context.
@@ -768,9 +767,9 @@ class BitmapCanvas extends EngineCanvas {
     // as well.
     assert(paint.shader == null,
         'Linear/Radial/SweepGradient and ImageShader not supported yet');
-    final Int32List colors = vertices._colors;
+    final Int32List? colors = vertices._colors;
     final ui.VertexMode mode = vertices._mode;
-    html.CanvasRenderingContext2D ctx = _canvasPool.context;
+    html.CanvasRenderingContext2D? ctx = _canvasPool.context;
     if (colors == null) {
       final Float32List positions = mode == ui.VertexMode.triangles
           ? vertices._positions
@@ -781,11 +780,11 @@ class BitmapCanvas extends EngineCanvas {
       _canvasPool.contextHandle
         ..fillStyle = null
         ..strokeStyle = colorToCssString(color);
-      _glRenderer.drawHairline(ctx, positions);
+      _glRenderer!.drawHairline(ctx, positions);
       restore();
       return;
     }
-    _glRenderer.drawVertices(ctx, _widthInBitmapPixels, _heightInBitmapPixels,
+    _glRenderer!.drawVertices(ctx, _widthInBitmapPixels, _heightInBitmapPixels,
         _canvasPool.currentTransform, vertices, blendMode, paint);
   }
 
@@ -809,7 +808,7 @@ class BitmapCanvas extends EngineCanvas {
     _drawPointsPaint.maskFilter = paint.maskFilter;
 
     _setUpPaint(_drawPointsPaint);
-    _canvasPool.drawPoints(pointMode, points, paint.strokeWidth / 2.0);
+    _canvasPool.drawPoints(pointMode, points, paint.strokeWidth! / 2.0);
     _tearDownPaint();
   }
 
@@ -821,7 +820,7 @@ class BitmapCanvas extends EngineCanvas {
   }
 }
 
-String _stringForBlendMode(ui.BlendMode blendMode) {
+String? _stringForBlendMode(ui.BlendMode? blendMode) {
   if (blendMode == null) {
     return null;
   }
@@ -887,7 +886,7 @@ String _stringForBlendMode(ui.BlendMode blendMode) {
   }
 }
 
-String _stringForStrokeCap(ui.StrokeCap strokeCap) {
+String? _stringForStrokeCap(ui.StrokeCap? strokeCap) {
   if (strokeCap == null) {
     return null;
   }
@@ -903,7 +902,7 @@ String _stringForStrokeCap(ui.StrokeCap strokeCap) {
 }
 
 String _stringForStrokeJoin(ui.StrokeJoin strokeJoin) {
-  assert(strokeJoin != null);
+  assert(strokeJoin != null); // ignore: unnecessary_null_comparison
   switch (strokeJoin) {
     case ui.StrokeJoin.round:
       return 'round';
@@ -924,7 +923,7 @@ String _stringForStrokeJoin(ui.StrokeJoin strokeJoin) {
 /// with a list of svg elements that provide clip-paths.
 List<html.Element> _clipContent(List<_SaveClipEntry> clipStack,
     html.HtmlElement content, ui.Offset offset, Matrix4 currentTransform) {
-  html.Element root, curElement;
+  html.Element? root, curElement;
   final List<html.Element> clipDefs = <html.Element>[];
   final int len = clipStack.length;
   for (int clipIndex = 0; clipIndex < len; clipIndex++) {
@@ -935,10 +934,10 @@ List<html.Element> _clipContent(List<_SaveClipEntry> clipStack,
     if (root == null) {
       root = newElement;
     } else {
-      domRenderer.append(curElement, newElement);
+      domRenderer.append(curElement!, newElement);
     }
     curElement = newElement;
-    final ui.Rect rect = entry.rect;
+    final ui.Rect? rect = entry.rect;
     Matrix4 newClipTransform = entry.currentTransform;
     if (rect != null) {
       final double clipOffsetX = rect.left;
@@ -951,7 +950,7 @@ List<html.Element> _clipContent(List<_SaveClipEntry> clipStack,
         ..height = '${rect.bottom - clipOffsetY}px';
       setElementTransform(curElement, newClipTransform.storage);
     } else if (entry.rrect != null) {
-      final ui.RRect roundRect = entry.rrect;
+      final ui.RRect roundRect = entry.rrect!;
       final String borderRadius =
           '${roundRect.tlRadiusX}px ${roundRect.trRadiusX}px '
           '${roundRect.brRadiusX}px ${roundRect.blRadiusX}px';
@@ -967,7 +966,7 @@ List<html.Element> _clipContent(List<_SaveClipEntry> clipStack,
       setElementTransform(curElement, newClipTransform.storage);
     } else if (entry.path != null) {
       curElement.style.transform = matrix4ToCssTransform(newClipTransform);
-      String svgClipPath = createSvgClipDef(curElement, entry.path);
+      String svgClipPath = createSvgClipDef(curElement as html.HtmlElement, entry.path!);
       final html.Element clipElement =
           html.Element.html(svgClipPath, treeSanitizer: _NullTreeSanitizer());
       clipDefs.add(clipElement);
@@ -986,8 +985,8 @@ List<html.Element> _clipContent(List<_SaveClipEntry> clipStack,
     curElement = reverseTransformDiv;
   }
 
-  root.style.position = 'absolute';
-  domRenderer.append(curElement, content);
+  root!.style.position = 'absolute';
+  domRenderer.append(curElement!, content);
   setElementTransform(
     content,
     transformWithOffset(currentTransform, offset).storage,
@@ -998,7 +997,7 @@ List<html.Element> _clipContent(List<_SaveClipEntry> clipStack,
 /// Converts a [maskFilter] to the value to be used on a `<canvas>`.
 ///
 /// Only supported in non-WebKit browsers.
-String _maskFilterToCanvasFilter(ui.MaskFilter maskFilter) {
+String _maskFilterToCanvasFilter(ui.MaskFilter? maskFilter) {
   assert(
     browserEngine != BrowserEngine.webkit,
     'WebKit (Safari) does not support `filter` canvas property.',
@@ -1027,7 +1026,7 @@ int _filterIdCounter = 0;
 // G' = g1*R + g2*G + g3*B + g4*A + g5
 // B' = b1*R + b2*G + b3*B + b4*A + b5
 // A' = a1*R + a2*G + a3*B + a4*A + a5
-String _srcInColorFilterToSvg(ui.Color color) {
+String _srcInColorFilterToSvg(ui.Color? color) {
   _filterIdCounter += 1;
   return '<svg width="0" height="0">'
       '<filter id="_fcf$_filterIdCounter" '
@@ -1044,7 +1043,7 @@ String _srcInColorFilterToSvg(ui.Color color) {
       '</filter></svg>';
 }
 
-String _srcOutColorFilterToSvg(ui.Color color) {
+String _srcOutColorFilterToSvg(ui.Color? color) {
   _filterIdCounter += 1;
   return '<svg width="0" height="0">'
       '<filter id="_fcf$_filterIdCounter" '
@@ -1056,7 +1055,7 @@ String _srcOutColorFilterToSvg(ui.Color color) {
       '</filter></svg>';
 }
 
-String _xorColorFilterToSvg(ui.Color color) {
+String _xorColorFilterToSvg(ui.Color? color) {
   _filterIdCounter += 1;
   return '<svg width="0" height="0">'
       '<filter id="_fcf$_filterIdCounter" '
@@ -1070,7 +1069,7 @@ String _xorColorFilterToSvg(ui.Color color) {
 
 // The source image and color are composited using :
 // result = k1 *in*in2 + k2*in + k3*in2 + k4.
-String _compositeColorFilterToSvg(ui.Color color, double k1, double k2, double k3 , double k4) {
+String _compositeColorFilterToSvg(ui.Color? color, double k1, double k2, double k3 , double k4) {
   _filterIdCounter += 1;
   return '<svg width="0" height="0">'
       '<filter id="_fcf$_filterIdCounter" '
@@ -1105,7 +1104,7 @@ String _modulateColorFilterToSvg(ui.Color color) {
 }
 
 // Uses feBlend element to blend source image with a color.
-String _blendColorFilterToSvg(ui.Color color, String feBlend,
+String _blendColorFilterToSvg(ui.Color? color, String? feBlend,
     {bool swapLayers = false}) {
   _filterIdCounter += 1;
   return '<svg width="0" height="0">'
diff --git a/lib/web_ui/lib/src/engine/browser_detection.dart b/lib/web_ui/lib/src/engine/browser_detection.dart
index 5c9b990670e07..20fdb7f0fb010 100644
--- a/lib/web_ui/lib/src/engine/browser_detection.dart
+++ b/lib/web_ui/lib/src/engine/browser_detection.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// The HTML engine used by the current browser.
@@ -28,7 +28,7 @@ enum BrowserEngine {
 }
 
 /// Lazily initialized current browser engine.
-BrowserEngine _browserEngine;
+late final BrowserEngine _browserEngine = _detectBrowserEngine();
 
 /// Override the value of [browserEngine].
 ///
@@ -36,16 +36,13 @@ BrowserEngine _browserEngine;
 /// app is running on.
 ///
 /// This is intended to be used for testing and debugging only.
-BrowserEngine debugBrowserEngineOverride;
+BrowserEngine? debugBrowserEngineOverride;
 
 /// Returns the [BrowserEngine] used by the current browser.
 ///
 /// This is used to implement browser-specific behavior.
 BrowserEngine get browserEngine {
-  if (debugBrowserEngineOverride != null) {
-    return debugBrowserEngineOverride;
-  }
-  return _browserEngine ??= _detectBrowserEngine();
+  return debugBrowserEngineOverride ?? _browserEngine;
 }
 
 BrowserEngine _detectBrowserEngine() {
@@ -99,17 +96,14 @@ enum OperatingSystem {
 }
 
 /// Lazily initialized current operating system.
-OperatingSystem _operatingSystem;
+late final OperatingSystem _operatingSystem = _detectOperatingSystem();
 
 /// Returns the [OperatingSystem] the current browsers works on.
 ///
 /// This is used to implement operating system specific behavior such as
 /// soft keyboards.
 OperatingSystem get operatingSystem {
-  if (debugOperatingSystemOverride != null) {
-    return debugOperatingSystemOverride;
-  }
-  return _operatingSystem ??= _detectOperatingSystem();
+  return debugOperatingSystemOverride ?? _operatingSystem;
 }
 
 /// Override the value of [operatingSystem].
@@ -118,7 +112,7 @@ OperatingSystem get operatingSystem {
 /// app is running on.
 ///
 /// This is intended to be used for testing and debugging only.
-OperatingSystem debugOperatingSystemOverride;
+OperatingSystem? debugOperatingSystemOverride;
 
 OperatingSystem _detectOperatingSystem() {
   final String platform = html.window.navigator.platform;
diff --git a/lib/web_ui/lib/src/engine/browser_location.dart b/lib/web_ui/lib/src/engine/browser_location.dart
index 854bf9ec6f5e0..2bbe9ec47c8d0 100644
--- a/lib/web_ui/lib/src/engine/browser_location.dart
+++ b/lib/web_ui/lib/src/engine/browser_location.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 // TODO(mdebbar): add other strategies.
@@ -115,7 +115,7 @@ class HashLocationStrategy extends LocationStrategy {
   /// `history.back` transition.
   Future<void> _waitForPopState() {
     final Completer<void> completer = Completer<void>();
-    ui.VoidCallback unsubscribe;
+    late ui.VoidCallback unsubscribe;
     unsubscribe = onPopState((_) {
       unsubscribe();
       completer.complete();
@@ -141,7 +141,7 @@ abstract class PlatformLocation {
 
   String get pathname;
   String get search;
-  String get hash;
+  String? get hash;
 
   void pushState(dynamic state, String title, String url);
   void replaceState(dynamic state, String title, String url);
diff --git a/lib/web_ui/lib/src/engine/canvas_pool.dart b/lib/web_ui/lib/src/engine/canvas_pool.dart
index 0be145972fdb7..dee43849534d1 100644
--- a/lib/web_ui/lib/src/engine/canvas_pool.dart
+++ b/lib/web_ui/lib/src/engine/canvas_pool.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Allocates and caches 0 or more canvas(s) for [BitmapCanvas].
@@ -21,17 +21,17 @@ part of engine;
 /// can be reused, [_CanvasPool] will move canvas(s) from pool to reusablePool
 /// to prevent reallocation.
 class _CanvasPool extends _SaveStackTracking {
-  html.CanvasRenderingContext2D _context;
-  ContextStateHandle _contextHandle;
+  html.CanvasRenderingContext2D? _context;
+  ContextStateHandle? _contextHandle;
   final int _widthInBitmapPixels, _heightInBitmapPixels;
   // List of canvases that have been allocated and used in this paint cycle.
-  List<html.CanvasElement> _activeCanvasList;
+  List<html.CanvasElement>? _activeCanvasList;
   // List of canvases available to reuse from prior paint cycle.
-  List<html.CanvasElement> _reusablePool;
+  List<html.CanvasElement>? _reusablePool;
   // Current canvas element or null if marked for lazy allocation.
-  html.CanvasElement _canvas;
+  html.CanvasElement? _canvas;
 
-  html.HtmlElement _rootElement;
+  html.HtmlElement? _rootElement;
   int _saveContextCount = 0;
   // Number of elements that have been added to flt-canvas.
   int _activeElementCount = 0;
@@ -39,12 +39,14 @@ class _CanvasPool extends _SaveStackTracking {
   _CanvasPool(this._widthInBitmapPixels, this._heightInBitmapPixels);
 
   html.CanvasRenderingContext2D get context {
-    if (_canvas == null) {
+    html.CanvasRenderingContext2D? ctx = _context;
+    if (ctx == null) {
       _createCanvas();
+      ctx = _context!;
       assert(_context != null);
       assert(_canvas != null);
     }
-    return _context;
+    return ctx;
   }
 
   ContextStateHandle get contextHandle {
@@ -53,7 +55,7 @@ class _CanvasPool extends _SaveStackTracking {
       assert(_context != null);
       assert(_canvas != null);
     }
-    return _contextHandle;
+    return _contextHandle!;
   }
 
   // Prevents active canvas to be used for rendering and prepares a new
@@ -67,9 +69,9 @@ class _CanvasPool extends _SaveStackTracking {
     // reset into pool.
     if (_canvas != null) {
       _restoreContextSave();
-      _contextHandle.reset();
+      _contextHandle!.reset();
       _activeCanvasList ??= [];
-      _activeCanvasList.add(_canvas);
+      _activeCanvasList!.add(_canvas!);
       _canvas = null;
       _context = null;
       _contextHandle = null;
@@ -84,8 +86,9 @@ class _CanvasPool extends _SaveStackTracking {
   void _createCanvas() {
     bool requiresClearRect = false;
     bool reused = false;
-    if (_reusablePool != null && _reusablePool.isNotEmpty) {
-      _canvas = _reusablePool.removeAt(0);
+    html.CanvasElement canvas;
+    if (_reusablePool != null && _reusablePool!.isNotEmpty) {
+      canvas = _canvas = _reusablePool!.removeAt(0);
       requiresClearRect = true;
       reused = true;
     } else {
@@ -99,19 +102,26 @@ class _CanvasPool extends _SaveStackTracking {
           _widthInBitmapPixels / EngineWindow.browserDevicePixelRatio;
       final double cssHeight =
           _heightInBitmapPixels / EngineWindow.browserDevicePixelRatio;
-      _canvas = html.CanvasElement(
+      canvas = html.CanvasElement(
         width: _widthInBitmapPixels,
         height: _heightInBitmapPixels,
       );
+      _canvas = canvas;
+
+      // Why is this null check here, even though we just allocated a canvas element above?
+      //
+      // On iOS Safari, if you alloate too many canvases, the browser will stop allocating them
+      // and return null instead. If that happens, we evict canvases from the cache, giving the
+      // browser more memory to allocate a new canvas.
       if (_canvas == null) {
         // Evict BitmapCanvas(s) and retry.
         _reduceCanvasMemoryUsage();
-        _canvas = html.CanvasElement(
+        canvas = html.CanvasElement(
           width: _widthInBitmapPixels,
           height: _heightInBitmapPixels,
         );
       }
-      _canvas.style
+      canvas.style
         ..position = 'absolute'
         ..width = '${cssWidth}px'
         ..height = '${cssHeight}px';
@@ -120,22 +130,22 @@ class _CanvasPool extends _SaveStackTracking {
     // Before appending canvas, check if canvas is already on rootElement. This
     // optimization prevents DOM .append call when a PersistentSurface is
     // reused. Reading lastChild is faster than append call.
-    if (_rootElement.lastChild != _canvas) {
-      _rootElement.append(_canvas);
+    if (_rootElement!.lastChild != canvas) {
+      _rootElement!.append(canvas);
     }
 
     if (_activeElementCount == 0) {
-      _canvas.style.zIndex = '-1';
+      canvas.style.zIndex = '-1';
     } else if (reused) {
       // If a canvas is the first element we set z-index = -1 to workaround
       // blink compositing bug. To make sure this does not leak when reused
       // reset z-index.
-      _canvas.style.removeProperty('z-index');
+      canvas.style.removeProperty('z-index');
     }
     ++_activeElementCount;
 
-    _context = _canvas.context2D;
-    _contextHandle = ContextStateHandle(this, _context);
+    final html.CanvasRenderingContext2D context = _context = canvas.context2D;
+    _contextHandle = ContextStateHandle(this, context);
     _initializeViewport(requiresClearRect);
     _replayClipStack();
   }
@@ -146,7 +156,7 @@ class _CanvasPool extends _SaveStackTracking {
 
     if (_canvas != null) {
       // Restore to the state where we have only applied the scaling.
-      html.CanvasRenderingContext2D ctx = _context;
+      html.CanvasRenderingContext2D? ctx = _context;
       if (ctx != null) {
         try {
           ctx.font = '';
@@ -168,8 +178,8 @@ class _CanvasPool extends _SaveStackTracking {
   }
 
   int _replaySingleSaveEntry(int clipDepth, Matrix4 prevTransform,
-      Matrix4 transform, List<_SaveClipEntry> clipStack) {
-    final html.CanvasRenderingContext2D ctx = _context;
+      Matrix4 transform, List<_SaveClipEntry>? clipStack) {
+    final html.CanvasRenderingContext2D ctx = context;
     if (clipStack != null) {
       for (int clipCount = clipStack.length;
           clipDepth < clipCount;
@@ -196,11 +206,11 @@ class _CanvasPool extends _SaveStackTracking {
           prevTransform = clipTimeTransform;
         }
         if (clipEntry.rect != null) {
-          _clipRect(ctx, clipEntry.rect);
+          _clipRect(ctx, clipEntry.rect!);
         } else if (clipEntry.rrect != null) {
-          _clipRRect(ctx, clipEntry.rrect);
+          _clipRRect(ctx, clipEntry.rrect!);
         } else if (clipEntry.path != null) {
-          _runPath(ctx, clipEntry.path);
+          _runPath(ctx, clipEntry.path as SurfacePath);
           ctx.clip();
         }
       }
@@ -223,7 +233,7 @@ class _CanvasPool extends _SaveStackTracking {
 
   void _replayClipStack() {
     // Replay save/clip stack on this canvas now.
-    html.CanvasRenderingContext2D ctx = _context;
+    html.CanvasRenderingContext2D ctx = context;
     int clipDepth = 0;
     Matrix4 prevTransform = Matrix4.identity();
     for (int saveStackIndex = 0, len = _saveStack.length;
@@ -244,9 +254,9 @@ class _CanvasPool extends _SaveStackTracking {
   void reuse() {
     if (_canvas != null) {
       _restoreContextSave();
-      _contextHandle.reset();
+      _contextHandle!.reset();
       _activeCanvasList ??= [];
-      _activeCanvasList.add(_canvas);
+      _activeCanvasList!.add(_canvas!);
       _context = null;
       _contextHandle = null;
     }
@@ -260,7 +270,7 @@ class _CanvasPool extends _SaveStackTracking {
 
   void endOfPaint() {
     if (_reusablePool != null) {
-      for (html.CanvasElement e in _reusablePool) {
+      for (html.CanvasElement e in _reusablePool!) {
         if (browserEngine == BrowserEngine.webkit) {
           e.width = e.height = 0;
         }
@@ -273,7 +283,7 @@ class _CanvasPool extends _SaveStackTracking {
 
   void _restoreContextSave() {
     while (_saveContextCount != 0) {
-      _context.restore();
+      _context!.restore();
       --_saveContextCount;
     }
   }
@@ -302,15 +312,16 @@ class _CanvasPool extends _SaveStackTracking {
   }
 
   void resetTransform() {
-    if (_canvas != null) {
-      _canvas.style.transformOrigin = '';
-      _canvas.style.transform = '';
+    final html.CanvasElement? canvas = _canvas;
+    if (canvas != null) {
+      canvas.style.transformOrigin = '';
+      canvas.style.transform = '';
     }
   }
 
   // Returns a data URI containing a representation of the image in this
   // canvas.
-  String toDataUrl() => _canvas.toDataUrl();
+  String toDataUrl() => _canvas!.toDataUrl();
 
   @override
   void save() {
@@ -423,7 +434,7 @@ class _CanvasPool extends _SaveStackTracking {
 
   void _clipRRect(html.CanvasRenderingContext2D ctx, ui.RRect rrect) {
     final ui.Path path = ui.Path()..addRRect(rrect);
-    _runPath(ctx, path);
+    _runPath(ctx, path as SurfacePath);
     ctx.clip();
   }
 
@@ -431,7 +442,7 @@ class _CanvasPool extends _SaveStackTracking {
     super.clipPath(path);
     if (_canvas != null) {
       html.CanvasRenderingContext2D ctx = context;
-      _runPath(ctx, path);
+      _runPath(ctx, path as SurfacePath);
       ctx.clip();
     }
   }
@@ -511,7 +522,7 @@ class _CanvasPool extends _SaveStackTracking {
         final PathCommand command = commands[c];
         switch (command.type) {
           case PathCommandTypes.bezierCurveTo:
-            final BezierCurveTo curve = command;
+            final BezierCurveTo curve = command as BezierCurveTo;
             ctx.bezierCurveTo(
                 curve.x1, curve.y1, curve.x2, curve.y2, curve.x3, curve.y3);
             break;
@@ -519,7 +530,7 @@ class _CanvasPool extends _SaveStackTracking {
             ctx.closePath();
             break;
           case PathCommandTypes.ellipse:
-            final Ellipse ellipse = command;
+            final Ellipse ellipse = command as Ellipse;
             if (c == 0) {
               // Ellipses that start a new path need to set start point,
               // otherwise it incorrectly uses last point.
@@ -536,25 +547,25 @@ class _CanvasPool extends _SaveStackTracking {
                 ellipse.anticlockwise);
             break;
           case PathCommandTypes.lineTo:
-            final LineTo lineTo = command;
+            final LineTo lineTo = command as LineTo;
             ctx.lineTo(lineTo.x, lineTo.y);
             break;
           case PathCommandTypes.moveTo:
-            final MoveTo moveTo = command;
+            final MoveTo moveTo = command as MoveTo;
             ctx.moveTo(moveTo.x, moveTo.y);
             break;
           case PathCommandTypes.rRect:
-            final RRectCommand rrectCommand = command;
+            final RRectCommand rrectCommand = command as RRectCommand;
             _RRectToCanvasRenderer(ctx)
                 .render(rrectCommand.rrect, startNewPath: false);
             break;
           case PathCommandTypes.rect:
-            final RectCommand rectCommand = command;
+            final RectCommand rectCommand = command as RectCommand;
             ctx.rect(rectCommand.x, rectCommand.y, rectCommand.width,
                 rectCommand.height);
             break;
           case PathCommandTypes.quadraticCurveTo:
-            final QuadraticCurveTo quadraticCurveTo = command;
+            final QuadraticCurveTo quadraticCurveTo = command as QuadraticCurveTo;
             ctx.quadraticCurveTo(quadraticCurveTo.x1, quadraticCurveTo.y1,
                 quadraticCurveTo.x2, quadraticCurveTo.y2);
             break;
@@ -565,45 +576,45 @@ class _CanvasPool extends _SaveStackTracking {
     }
   }
 
-  void drawRect(ui.Rect rect, ui.PaintingStyle style) {
+  void drawRect(ui.Rect rect, ui.PaintingStyle? style) {
     context.beginPath();
     context.rect(rect.left, rect.top, rect.width, rect.height);
     contextHandle.paint(style);
   }
 
-  void drawRRect(ui.RRect roundRect, ui.PaintingStyle style) {
+  void drawRRect(ui.RRect roundRect, ui.PaintingStyle? style) {
     _RRectToCanvasRenderer(context).render(roundRect);
     contextHandle.paint(style);
   }
 
-  void drawDRRect(ui.RRect outer, ui.RRect inner, ui.PaintingStyle style) {
+  void drawDRRect(ui.RRect outer, ui.RRect inner, ui.PaintingStyle? style) {
     _RRectRenderer renderer = _RRectToCanvasRenderer(context);
     renderer.render(outer);
     renderer.render(inner, startNewPath: false, reverse: true);
     contextHandle.paint(style);
   }
 
-  void drawOval(ui.Rect rect, ui.PaintingStyle style) {
+  void drawOval(ui.Rect rect, ui.PaintingStyle? style) {
     context.beginPath();
     DomRenderer.ellipse(context, rect.center.dx, rect.center.dy, rect.width / 2,
         rect.height / 2, 0, 0, 2.0 * math.pi, false);
     contextHandle.paint(style);
   }
 
-  void drawCircle(ui.Offset c, double radius, ui.PaintingStyle style) {
+  void drawCircle(ui.Offset c, double radius, ui.PaintingStyle? style) {
     context.beginPath();
     DomRenderer.ellipse(context, c.dx, c.dy, radius, radius, 0, 0, 2.0 * math.pi, false);
     contextHandle.paint(style);
   }
 
-  void drawPath(ui.Path path, ui.PaintingStyle style) {
-    _runPath(context, path);
+  void drawPath(ui.Path path, ui.PaintingStyle? style) {
+    _runPath(context, path as SurfacePath);
     contextHandle.paintPath(style, path.fillType);
   }
 
   void drawShadow(ui.Path path, ui.Color color, double elevation,
       bool transparentOccluder) {
-    final SurfaceShadowData shadow = computeShadow(path.getBounds(), elevation);
+    final SurfaceShadowData? shadow = computeShadow(path.getBounds(), elevation);
     if (shadow != null) {
       // On April 2020 Web canvas 2D did not support shadow color alpha. So
       // instead we apply alpha separately using globalAlpha, then paint a
@@ -654,7 +665,7 @@ class _CanvasPool extends _SaveStackTracking {
         context.shadowOffsetX = shadow.offset.dx;
         context.shadowOffsetY = shadow.offset.dy;
       }
-      _runPath(context, path);
+      _runPath(context, path as SurfacePath);
       context.fill();
 
       // This also resets globalAlpha and shadow attributes. See:
@@ -671,14 +682,14 @@ class _CanvasPool extends _SaveStackTracking {
     // into thinking that this canvas has a zero size so it doesn't count it
     // towards the threshold.
     if (browserEngine == BrowserEngine.webkit && _canvas != null) {
-      _canvas.width = _canvas.height = 0;
+      _canvas!.width = _canvas!.height = 0;
     }
     _clearActiveCanvasList();
   }
 
   void _clearActiveCanvasList() {
     if (_activeCanvasList != null) {
-      for (html.CanvasElement c in _activeCanvasList) {
+      for (html.CanvasElement c in _activeCanvasList!) {
         if (browserEngine == BrowserEngine.webkit) {
           c.width = c.height = 0;
         }
@@ -699,16 +710,16 @@ class ContextStateHandle {
   final _CanvasPool _canvasPool;
 
   ContextStateHandle(this._canvasPool, this.context);
-  ui.BlendMode _currentBlendMode = ui.BlendMode.srcOver;
-  ui.StrokeCap _currentStrokeCap = ui.StrokeCap.butt;
-  ui.StrokeJoin _currentStrokeJoin = ui.StrokeJoin.miter;
+  ui.BlendMode? _currentBlendMode = ui.BlendMode.srcOver;
+  ui.StrokeCap? _currentStrokeCap = ui.StrokeCap.butt;
+  ui.StrokeJoin? _currentStrokeJoin = ui.StrokeJoin.miter;
   // Fill style and stroke style are Object since they can have a String or
   // shader object such as a gradient.
-  Object _currentFillStyle;
-  Object _currentStrokeStyle;
+  Object? _currentFillStyle;
+  Object? _currentStrokeStyle;
   double _currentLineWidth = 1.0;
 
-  set blendMode(ui.BlendMode blendMode) {
+  set blendMode(ui.BlendMode? blendMode) {
     if (blendMode != _currentBlendMode) {
       _currentBlendMode = blendMode;
       context.globalCompositeOperation =
@@ -716,11 +727,11 @@ class ContextStateHandle {
     }
   }
 
-  set strokeCap(ui.StrokeCap strokeCap) {
+  set strokeCap(ui.StrokeCap? strokeCap) {
     strokeCap ??= ui.StrokeCap.butt;
     if (strokeCap != _currentStrokeCap) {
       _currentStrokeCap = strokeCap;
-      context.lineCap = _stringForStrokeCap(strokeCap);
+      context.lineCap = _stringForStrokeCap(strokeCap)!;
     }
   }
 
@@ -731,7 +742,7 @@ class ContextStateHandle {
     }
   }
 
-  set strokeJoin(ui.StrokeJoin strokeJoin) {
+  set strokeJoin(ui.StrokeJoin? strokeJoin) {
     strokeJoin ??= ui.StrokeJoin.miter;
     if (strokeJoin != _currentStrokeJoin) {
       _currentStrokeJoin = strokeJoin;
@@ -739,22 +750,22 @@ class ContextStateHandle {
     }
   }
 
-  set fillStyle(Object colorOrGradient) {
+  set fillStyle(Object? colorOrGradient) {
     if (!identical(colorOrGradient, _currentFillStyle)) {
       _currentFillStyle = colorOrGradient;
       context.fillStyle = colorOrGradient;
     }
   }
 
-  set strokeStyle(Object colorOrGradient) {
+  set strokeStyle(Object? colorOrGradient) {
     if (!identical(colorOrGradient, _currentStrokeStyle)) {
       _currentStrokeStyle = colorOrGradient;
       context.strokeStyle = colorOrGradient;
     }
   }
 
-  ui.MaskFilter _currentFilter;
-  SurfacePaintData _lastUsedPaint;
+  ui.MaskFilter? _currentFilter;
+  SurfacePaintData? _lastUsedPaint;
 
   /// The painting state.
   ///
@@ -785,13 +796,13 @@ class ContextStateHandle {
     strokeJoin = paint.strokeJoin;
 
     if (paint.shader != null) {
-      final EngineGradient engineShader = paint.shader;
+      final EngineGradient engineShader = paint.shader as EngineGradient;
       final Object paintStyle =
           engineShader.createPaintStyle(_canvasPool.context);
       fillStyle = paintStyle;
       strokeStyle = paintStyle;
     } else if (paint.color != null) {
-      final String colorString = colorToCssString(paint.color);
+      final String? colorString = colorToCssString(paint.color);
       fillStyle = colorString;
       strokeStyle = colorString;
     } else {
@@ -799,7 +810,7 @@ class ContextStateHandle {
       strokeStyle = '';
     }
 
-    final ui.MaskFilter maskFilter = paint?.maskFilter;
+    final ui.MaskFilter? maskFilter = paint.maskFilter;
     if (!_renderMaskFilterForWebkit) {
       if (_currentFilter != maskFilter) {
         _currentFilter = maskFilter;
@@ -815,11 +826,11 @@ class ContextStateHandle {
       if (maskFilter != null) {
         context.save();
         context.shadowBlur = convertSigmaToRadius(maskFilter.webOnlySigma);
-        if (paint?.color != null) {
+        if (paint.color != null) {
           // Shadow color must be fully opaque.
-          context.shadowColor = colorToCssString(paint.color.withAlpha(255));
+          context.shadowColor = colorToCssString(paint.color!.withAlpha(255))!;
         } else {
-          context.shadowColor = colorToCssString(const ui.Color(0xFF000000));
+          context.shadowColor = colorToCssString(const ui.Color(0xFF000000))!;
         }
 
         // On the web a shadow must always be painted together with the shape
@@ -865,7 +876,7 @@ class ContextStateHandle {
       _debugIsPaintSetUp = false;
     }
 
-    final ui.MaskFilter maskFilter = _lastUsedPaint?.maskFilter;
+    final ui.MaskFilter? maskFilter = _lastUsedPaint?.maskFilter;
     if (maskFilter != null && _renderMaskFilterForWebkit) {
       // On Safari (WebKit) we use a translated shadow to emulate
       // MaskFilter.blur. We use restore to undo the translation and
@@ -874,7 +885,7 @@ class ContextStateHandle {
     }
   }
 
-  void paint(ui.PaintingStyle style) {
+  void paint(ui.PaintingStyle? style) {
     if (style == ui.PaintingStyle.stroke) {
       context.stroke();
     } else {
@@ -882,7 +893,7 @@ class ContextStateHandle {
     }
   }
 
-  void paintPath(ui.PaintingStyle style, ui.PathFillType pathFillType) {
+  void paintPath(ui.PaintingStyle? style, ui.PathFillType pathFillType) {
     if (style == ui.PaintingStyle.stroke) {
       context.stroke();
     } else {
@@ -927,7 +938,7 @@ class _SaveStackTracking {
 
   /// The stack that maintains clipping operations used when text is painted
   /// onto bitmap canvas but is composited as separate element.
-  List<_SaveClipEntry> _clipStack;
+  List<_SaveClipEntry>? _clipStack;
 
   /// Returns whether there are active clipping regions on the canvas.
   bool get isClipped => _clipStack != null;
@@ -951,7 +962,7 @@ class _SaveStackTracking {
     _saveStack.add(_SaveStackEntry(
       transform: _currentTransform.clone(),
       clipStack:
-          _clipStack == null ? null : List<_SaveClipEntry>.from(_clipStack),
+          _clipStack == null ? null : List<_SaveClipEntry>.from(_clipStack!),
     ));
   }
 
@@ -1004,20 +1015,20 @@ class _SaveStackTracking {
   @mustCallSuper
   void clipRect(ui.Rect rect) {
     _clipStack ??= <_SaveClipEntry>[];
-    _clipStack.add(_SaveClipEntry.rect(rect, _currentTransform.clone()));
+    _clipStack!.add(_SaveClipEntry.rect(rect, _currentTransform.clone()));
   }
 
   /// Adds a round rectangle to clipping stack.
   @mustCallSuper
   void clipRRect(ui.RRect rrect) {
     _clipStack ??= <_SaveClipEntry>[];
-    _clipStack.add(_SaveClipEntry.rrect(rrect, _currentTransform.clone()));
+    _clipStack!.add(_SaveClipEntry.rrect(rrect, _currentTransform.clone()));
   }
 
   /// Adds a path to clipping stack.
   @mustCallSuper
   void clipPath(ui.Path path) {
     _clipStack ??= <_SaveClipEntry>[];
-    _clipStack.add(_SaveClipEntry.path(path, _currentTransform.clone()));
+    _clipStack!.add(_SaveClipEntry.path(path, _currentTransform.clone()));
   }
 }
diff --git a/lib/web_ui/lib/src/engine/clipboard.dart b/lib/web_ui/lib/src/engine/clipboard.dart
index cc5ed5532c96c..f9446b68a2e60 100644
--- a/lib/web_ui/lib/src/engine/clipboard.dart
+++ b/lib/web_ui/lib/src/engine/clipboard.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Handles clipboard related platform messages.
@@ -16,16 +16,16 @@ class ClipboardMessageHandler {
 
   /// Handles the platform message which stores the given text to the clipboard.
   void setDataMethodCall(
-      MethodCall methodCall, ui.PlatformMessageResponseCallback callback) {
+      MethodCall methodCall, ui.PlatformMessageResponseCallback? callback) {
     const MethodCodec codec = JSONMethodCodec();
     bool errorEnvelopeEncoded = false;
     _copyToClipboardStrategy
         .setData(methodCall.arguments['text'])
         .then((bool success) {
       if (success) {
-        callback(codec.encodeSuccessEnvelope(true));
+        callback!(codec.encodeSuccessEnvelope(true));
       } else {
-        callback(codec.encodeErrorEnvelope(
+        callback!(codec.encodeErrorEnvelope(
             code: 'copy_fail', message: 'Clipboard.setData failed'));
         errorEnvelopeEncoded = true;
       }
@@ -33,21 +33,21 @@ class ClipboardMessageHandler {
       // Don't encode a duplicate reply if we already failed and an error
       // was already encoded.
       if (!errorEnvelopeEncoded) {
-        callback(codec.encodeErrorEnvelope(
+        callback!(codec.encodeErrorEnvelope(
             code: 'copy_fail', message: 'Clipboard.setData failed'));
       }
     });
   }
 
   /// Handles the platform message which retrieves text data from the clipboard.
-  void getDataMethodCall(ui.PlatformMessageResponseCallback callback) {
+  void getDataMethodCall(ui.PlatformMessageResponseCallback? callback) {
     const MethodCodec codec = JSONMethodCodec();
     _pasteFromClipboardStrategy.getData().then((String data) {
       final Map<String, dynamic> map = <String, dynamic>{'text': data};
-      callback(codec.encodeSuccessEnvelope(map));
+      callback!(codec.encodeSuccessEnvelope(map));
     }).catchError((dynamic error) {
       print('Could not get text from clipboard: $error');
-      callback(codec.encodeErrorEnvelope(
+      callback!(codec.encodeErrorEnvelope(
           code: 'paste_fail', message: 'Clipboard.getData failed'));
     });
   }
@@ -62,13 +62,17 @@ class ClipboardMessageHandler {
   }
 }
 
+bool _unsafeIsNull(dynamic object) {
+  return object == null;
+}
+
 /// Provides functionality for writing text to clipboard.
 ///
 /// A concrete implementation is picked at runtime based on the available
 /// APIs and the browser.
 abstract class CopyToClipboardStrategy {
   factory CopyToClipboardStrategy() {
-    return (html.window.navigator.clipboard?.writeText != null)
+    return !_unsafeIsNull(html.window.navigator.clipboard)
         ? ClipboardAPICopyStrategy()
         : ExecCommandCopyStrategy();
   }
@@ -78,7 +82,7 @@ abstract class CopyToClipboardStrategy {
   /// Returns `true` for a successful action.
   ///
   /// Returns `false` for an uncessful action or when there is an excaption.
-  Future<bool> setData(String text);
+  Future<bool> setData(String? text);
 }
 
 /// Provides functionality for reading text from clipboard.
@@ -88,7 +92,7 @@ abstract class CopyToClipboardStrategy {
 abstract class PasteFromClipboardStrategy {
   factory PasteFromClipboardStrategy() {
     return (browserEngine == BrowserEngine.firefox ||
-            html.window.navigator.clipboard?.readText == null)
+            _unsafeIsNull(html.window.navigator.clipboard))
         ? ExecCommandPasteStrategy()
         : ClipboardAPIPasteStrategy();
   }
@@ -103,11 +107,11 @@ abstract class PasteFromClipboardStrategy {
 /// See: https://developer.mozilla.org/en-US/docs/Web/API/Clipboard_API
 class ClipboardAPICopyStrategy implements CopyToClipboardStrategy {
   @override
-  Future<bool> setData(String text) async {
+  Future<bool> setData(String? text) async {
     try {
-      await html.window.navigator.clipboard.writeText(text);
-    } catch (e) {
-      print('copy is not successful ${e.message}');
+      await html.window.navigator.clipboard.writeText(text!);
+    } catch (error) {
+      print('copy is not successful $error');
       return Future.value(false);
     }
     return Future.value(true);
@@ -130,11 +134,11 @@ class ClipboardAPIPasteStrategy implements PasteFromClipboardStrategy {
 /// Provides a fallback strategy for browsers which does not support ClipboardAPI.
 class ExecCommandCopyStrategy implements CopyToClipboardStrategy {
   @override
-  Future<bool> setData(String text) {
+  Future<bool> setData(String? text) {
     return Future.value(_setDataSync(text));
   }
 
-  bool _setDataSync(String text) {
+  bool _setDataSync(String? text) {
     // Copy content to clipboard with execCommand.
     // See: https://developers.google.com/web/updates/2015/04/cut-and-copy-commands
     final html.TextAreaElement tempTextArea = _appendTemporaryTextArea();
@@ -147,8 +151,8 @@ class ExecCommandCopyStrategy implements CopyToClipboardStrategy {
       if (!result) {
         print('copy is not successful');
       }
-    } catch (e) {
-      print('copy is not successful ${e.message}');
+    } catch (error) {
+      print('copy is not successful $error');
     } finally {
       _removeTemporaryTextArea(tempTextArea);
     }
@@ -167,13 +171,13 @@ class ExecCommandCopyStrategy implements CopyToClipboardStrategy {
       ..backgroundColor = 'transparent'
       ..background = 'transparent';
 
-    html.document.body.append(tempElement);
+    html.document.body!.append(tempElement);
 
     return tempElement;
   }
 
   void _removeTemporaryTextArea(html.HtmlElement element) {
-    element?.remove();
+    element.remove();
   }
 }
 
diff --git a/lib/web_ui/lib/src/engine/color_filter.dart b/lib/web_ui/lib/src/engine/color_filter.dart
index c674ea807e3a6..4f8c82b77821e 100644
--- a/lib/web_ui/lib/src/engine/color_filter.dart
+++ b/lib/web_ui/lib/src/engine/color_filter.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A description of a color filter to apply when drawing a shape or compositing
@@ -109,9 +109,9 @@ class EngineColorFilter implements ui.ColorFilter {
         _matrix = null,
         _type = _TypeSrgbToLinearGamma;
 
-  final ui.Color _color;
-  final ui.BlendMode _blendMode;
-  final List<double> _matrix;
+  final ui.Color? _color;
+  final ui.BlendMode? _blendMode;
+  final List<double>? _matrix;
   final int _type;
 
   // The type of SkColorFilter class to create for Skia.
@@ -137,7 +137,7 @@ class EngineColorFilter implements ui.ColorFilter {
     return _color == typedOther._color && _blendMode == typedOther._blendMode;
   }
 
-  SkColorFilter _toSkColorFilter() {
+  SkColorFilter? _toSkColorFilter() {
     switch (_type) {
       case _TypeMode:
         if (_color == null || _blendMode == null) {
@@ -148,7 +148,7 @@ class EngineColorFilter implements ui.ColorFilter {
         if (_matrix == null) {
           return null;
         }
-        assert(_matrix.length == 20, 'Color Matrix must have 20 entries.');
+        assert(_matrix!.length == 20, 'Color Matrix must have 20 entries.');
         return SkColorFilter.matrix(this);
       case _TypeLinearToSrgbGamma:
         return SkColorFilter.linearToSrgbGamma(this);
diff --git a/lib/web_ui/lib/src/engine/compositor/canvas.dart b/lib/web_ui/lib/src/engine/compositor/canvas.dart
index dac20416d46dc..3377aa8a8379f 100644
--- a/lib/web_ui/lib/src/engine/compositor/canvas.dart
+++ b/lib/web_ui/lib/src/engine/compositor/canvas.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A Dart wrapper around Skia's SKCanvas.
@@ -11,16 +11,16 @@ class SkCanvas {
 
   SkCanvas(this.skCanvas);
 
-  int get saveCount => skCanvas.callMethod('getSaveCount');
+  int? get saveCount => skCanvas.callMethod('getSaveCount');
 
   void clear(ui.Color color) {
     setSharedSkColor1(color);
-    skCanvas.callMethod('clear', <js.JsObject>[sharedSkColor1]);
+    skCanvas.callMethod('clear', <js.JsObject?>[sharedSkColor1]);
   }
 
   void clipPath(ui.Path path, bool doAntiAlias) {
-    final SkPath skPath = path;
-    final js.JsObject intersectClipOp = canvasKit['ClipOp']['Intersect'];
+    final SkPath skPath = path as SkPath;
+    final js.JsObject? intersectClipOp = canvasKit['ClipOp']['Intersect'];
     skCanvas.callMethod('clipPath', <dynamic>[
       skPath._skPath,
       intersectClipOp,
@@ -29,7 +29,7 @@ class SkCanvas {
   }
 
   void clipRRect(ui.RRect rrect, bool doAntiAlias) {
-    final js.JsObject intersectClipOp = canvasKit['ClipOp']['Intersect'];
+    final js.JsObject? intersectClipOp = canvasKit['ClipOp']['Intersect'];
     skCanvas.callMethod('clipRRect', <dynamic>[
       makeSkRRect(rrect),
       intersectClipOp,
@@ -38,7 +38,7 @@ class SkCanvas {
   }
 
   void clipRect(ui.Rect rect, ui.ClipOp clipOp, bool doAntiAlias) {
-    js.JsObject skClipOp;
+    js.JsObject? skClipOp;
     switch (clipOp) {
       case ui.ClipOp.difference:
         skClipOp = canvasKit['ClipOp']['Difference'];
@@ -74,10 +74,10 @@ class SkCanvas {
     ui.Image atlas,
     Float32List rstTransforms,
     Float32List rects,
-    js.JsArray<Float32List> colors,
+    js.JsArray<Float32List>? colors,
     ui.BlendMode blendMode,
   ) {
-    final SkImage skAtlas = atlas;
+    final SkImage skAtlas = atlas as SkImage;
     skCanvas.callMethod('drawAtlas', <dynamic>[
       skAtlas.skImage,
       rects,
@@ -105,7 +105,7 @@ class SkCanvas {
   }
 
   void drawDRRect(ui.RRect outer, ui.RRect inner, SkPaint paint) {
-    skCanvas.callMethod('drawDRRect', <js.JsObject>[
+    skCanvas.callMethod('drawDRRect', <js.JsObject?>[
       makeSkRRect(outer),
       makeSkRRect(inner),
       paint.skiaObject,
@@ -113,7 +113,7 @@ class SkCanvas {
   }
 
   void drawImage(ui.Image image, ui.Offset offset, SkPaint paint) {
-    final SkImage skImage = image;
+    final SkImage skImage = image as SkImage;
     skCanvas.callMethod('drawImage', <dynamic>[
       skImage.skImage,
       offset.dx,
@@ -123,7 +123,7 @@ class SkCanvas {
   }
 
   void drawImageRect(ui.Image image, ui.Rect src, ui.Rect dst, SkPaint paint) {
-    final SkImage skImage = image;
+    final SkImage skImage = image as SkImage;
     skCanvas.callMethod('drawImageRect', <dynamic>[
       skImage.skImage,
       makeSkRect(src),
@@ -135,7 +135,7 @@ class SkCanvas {
 
   void drawImageNine(
       ui.Image image, ui.Rect center, ui.Rect dst, SkPaint paint) {
-    final SkImage skImage = image;
+    final SkImage skImage = image as SkImage;
     skCanvas.callMethod('drawImageNine', <dynamic>[
       skImage.skImage,
       makeSkRect(center),
@@ -155,18 +155,18 @@ class SkCanvas {
   }
 
   void drawOval(ui.Rect rect, SkPaint paint) {
-    skCanvas.callMethod('drawOval', <js.JsObject>[
+    skCanvas.callMethod('drawOval', <js.JsObject?>[
       makeSkRect(rect),
       paint.skiaObject,
     ]);
   }
 
   void drawPaint(SkPaint paint) {
-    skCanvas.callMethod('drawPaint', <js.JsObject>[paint.skiaObject]);
+    skCanvas.callMethod('drawPaint', <js.JsObject?>[paint.skiaObject]);
   }
 
   void drawParagraph(ui.Paragraph paragraph, ui.Offset offset) {
-    final SkParagraph skParagraph = paragraph;
+    final SkParagraph skParagraph = paragraph as SkParagraph;
     skCanvas.callMethod('drawParagraph', <dynamic>[
       skParagraph.skParagraph,
       offset.dx,
@@ -175,20 +175,20 @@ class SkCanvas {
   }
 
   void drawPath(ui.Path path, SkPaint paint) {
-    final js.JsObject skPaint = paint.skiaObject;
-    final SkPath enginePath = path;
-    final js.JsObject skPath = enginePath._skPath;
-    skCanvas.callMethod('drawPath', <js.JsObject>[skPath, skPaint]);
+    final js.JsObject? skPaint = paint.skiaObject;
+    final SkPath enginePath = path as SkPath;
+    final js.JsObject? skPath = enginePath._skPath;
+    skCanvas.callMethod('drawPath', <js.JsObject?>[skPath, skPaint]);
   }
 
   void drawPicture(ui.Picture picture) {
-    final SkPicture skPicture = picture;
-    skCanvas.callMethod('drawPicture', <js.JsObject>[skPicture.skPicture]);
+    final SkPicture skPicture = picture as SkPicture;
+    skCanvas.callMethod('drawPicture', <js.JsObject?>[skPicture.skPicture]);
   }
 
   // TODO(hterkelsen): https://github.com/flutter/flutter/issues/58824
   void drawPoints(SkPaint paint, ui.PointMode pointMode,
-      js.JsArray<js.JsArray<double>> points) {
+      js.JsArray<js.JsArray<double>>? points) {
     skCanvas.callMethod('drawPoints', <dynamic>[
       makeSkPointMode(pointMode),
       points,
@@ -197,7 +197,7 @@ class SkCanvas {
   }
 
   void drawRRect(ui.RRect rrect, SkPaint paint) {
-    skCanvas.callMethod('drawRRect', <js.JsObject>[
+    skCanvas.callMethod('drawRRect', <js.JsObject?>[
       makeSkRRect(rrect),
       paint.skiaObject,
     ]);
@@ -205,20 +205,20 @@ class SkCanvas {
 
   void drawRect(ui.Rect rect, SkPaint paint) {
     final js.JsObject skRect = makeSkRect(rect);
-    final js.JsObject skPaint = paint.skiaObject;
-    skCanvas.callMethod('drawRect', <js.JsObject>[skRect, skPaint]);
+    final js.JsObject? skPaint = paint.skiaObject;
+    skCanvas.callMethod('drawRect', <js.JsObject?>[skRect, skPaint]);
   }
 
   void drawShadow(ui.Path path, ui.Color color, double elevation,
       bool transparentOccluder) {
-    drawSkShadow(skCanvas, path, color, elevation, transparentOccluder,
+    drawSkShadow(skCanvas, path as SkPath, color, elevation, transparentOccluder,
         ui.window.devicePixelRatio);
   }
 
   void drawVertices(
       ui.Vertices vertices, ui.BlendMode blendMode, SkPaint paint) {
-    SkVertices skVertices = vertices;
-    skCanvas.callMethod('drawVertices', <js.JsObject>[
+    SkVertices skVertices = vertices as SkVertices;
+    skCanvas.callMethod('drawVertices', <js.JsObject?>[
       skVertices.skVertices,
       makeSkBlendMode(blendMode),
       paint.skiaObject
@@ -229,8 +229,8 @@ class SkCanvas {
     skCanvas.callMethod('restore');
   }
 
-  void restoreToCount(int count) {
-    skCanvas.callMethod('restoreToCount', <int>[count]);
+  void restoreToCount(int? count) {
+    skCanvas.callMethod('restoreToCount', <int?>[count]);
   }
 
   void rotate(double radians) {
@@ -238,24 +238,24 @@ class SkCanvas {
         .callMethod('rotate', <double>[radians * 180.0 / math.pi, 0.0, 0.0]);
   }
 
-  int save() {
+  int? save() {
     return skCanvas.callMethod('save');
   }
 
   void saveLayer(ui.Rect bounds, SkPaint paint) {
-    assert(bounds != null, 'Use saveLayerWithoutBounds');
-    skCanvas.callMethod('saveLayer', <js.JsObject>[
+    assert(bounds != null, 'Use saveLayerWithoutBounds'); // ignore: unnecessary_null_comparison
+    skCanvas.callMethod('saveLayer', <js.JsObject?>[
       makeSkRect(bounds),
       paint.skiaObject,
     ]);
   }
 
   void saveLayerWithoutBounds(SkPaint paint) {
-    skCanvas.callMethod('saveLayer', <js.JsObject>[paint.skiaObject]);
+    skCanvas.callMethod('saveLayer', <js.JsObject?>[paint.skiaObject]);
   }
 
   void saveLayerWithFilter(ui.Rect bounds, ui.ImageFilter filter) {
-    final SkImageFilter skImageFilter = filter;
+    final SkImageFilter skImageFilter = filter as SkImageFilter;
     return skCanvas.callMethod(
       'saveLayer',
       <dynamic>[
@@ -275,7 +275,7 @@ class SkCanvas {
     skCanvas.callMethod('skew', <double>[sx, sy]);
   }
 
-  void transform(Float32List matrix4) {
+  void transform(Float32List? matrix4) {
     skCanvas.callMethod(
         'concat', <js.JsArray<double>>[makeSkMatrixFromFloat32(matrix4)]);
   }
diff --git a/lib/web_ui/lib/src/engine/compositor/canvas_kit_canvas.dart b/lib/web_ui/lib/src/engine/compositor/canvas_kit_canvas.dart
index fda4b67f9fde0..50e3373486402 100644
--- a/lib/web_ui/lib/src/engine/compositor/canvas_kit_canvas.dart
+++ b/lib/web_ui/lib/src/engine/compositor/canvas_kit_canvas.dart
@@ -2,21 +2,21 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// An implementation of [ui.Canvas] that is backed by a CanvasKit canvas.
 class CanvasKitCanvas implements ui.Canvas {
-  final SkCanvas _canvas;
+  final SkCanvas? _canvas;
 
-  factory CanvasKitCanvas(ui.PictureRecorder recorder, [ui.Rect cullRect]) {
-    assert(recorder != null);
+  factory CanvasKitCanvas(ui.PictureRecorder recorder, [ui.Rect? cullRect]) {
+    assert(recorder != null); // ignore: unnecessary_null_comparison
     if (recorder.isRecording) {
       throw ArgumentError(
           '"recorder" must not already be associated with another Canvas.');
     }
     cullRect ??= ui.Rect.largest;
-    final SkPictureRecorder skRecorder = recorder;
+    final SkPictureRecorder skRecorder = recorder as SkPictureRecorder;
     return CanvasKitCanvas._(skRecorder.beginRecording(cullRect));
   }
 
@@ -24,12 +24,12 @@ class CanvasKitCanvas implements ui.Canvas {
 
   @override
   void save() {
-    _canvas.save();
+    _canvas!.save();
   }
 
   @override
-  void saveLayer(ui.Rect bounds, ui.Paint paint) {
-    assert(paint != null);
+  void saveLayer(ui.Rect? bounds, ui.Paint paint) {
+    assert(paint != null); // ignore: unnecessary_null_comparison
     if (bounds == null) {
       _saveLayerWithoutBounds(paint);
     } else {
@@ -39,48 +39,48 @@ class CanvasKitCanvas implements ui.Canvas {
   }
 
   void _saveLayerWithoutBounds(ui.Paint paint) {
-    _canvas.saveLayerWithoutBounds(paint);
+    _canvas!.saveLayerWithoutBounds(paint as SkPaint);
   }
 
   void _saveLayer(ui.Rect bounds, ui.Paint paint) {
-    _canvas.saveLayer(bounds, paint);
+    _canvas!.saveLayer(bounds, paint as SkPaint);
   }
 
   @override
   void restore() {
-    _canvas.restore();
+    _canvas!.restore();
   }
 
   @override
   int getSaveCount() {
-    return _canvas.saveCount;
+    return _canvas!.saveCount!;
   }
 
   @override
   void translate(double dx, double dy) {
-    _canvas.translate(dx, dy);
+    _canvas!.translate(dx, dy);
   }
 
   @override
-  void scale(double sx, [double sy]) => _scale(sx, sy ?? sx);
+  void scale(double sx, [double? sy]) => _scale(sx, sy ?? sx);
 
   void _scale(double sx, double sy) {
-    _canvas.scale(sx, sy);
+    _canvas!.scale(sx, sy);
   }
 
   @override
   void rotate(double radians) {
-    _canvas.rotate(radians);
+    _canvas!.rotate(radians);
   }
 
   @override
   void skew(double sx, double sy) {
-    _canvas.skew(sx, sy);
+    _canvas!.skew(sx, sy);
   }
 
   @override
   void transform(Float64List matrix4) {
-    assert(matrix4 != null);
+    assert(matrix4 != null); // ignore: unnecessary_null_comparison
     if (matrix4.length != 16) {
       throw ArgumentError('"matrix4" must have 16 entries.');
     }
@@ -88,234 +88,240 @@ class CanvasKitCanvas implements ui.Canvas {
   }
 
   void _transform(Float32List matrix4) {
-    _canvas.transform(matrix4);
+    _canvas!.transform(matrix4);
   }
 
   @override
   void clipRect(ui.Rect rect,
       {ui.ClipOp clipOp = ui.ClipOp.intersect, bool doAntiAlias = true}) {
     assert(rectIsValid(rect));
-    assert(clipOp != null);
-    assert(doAntiAlias != null);
+    assert(clipOp != null); // ignore: unnecessary_null_comparison
+    assert(doAntiAlias != null); // ignore: unnecessary_null_comparison
     _clipRect(rect, clipOp, doAntiAlias);
   }
 
   void _clipRect(ui.Rect rect, ui.ClipOp clipOp, bool doAntiAlias) {
-    _canvas.clipRect(rect, clipOp, doAntiAlias);
+    _canvas!.clipRect(rect, clipOp, doAntiAlias);
   }
 
   @override
   void clipRRect(ui.RRect rrect, {bool doAntiAlias = true}) {
     assert(rrectIsValid(rrect));
-    assert(doAntiAlias != null);
+    assert(doAntiAlias != null); // ignore: unnecessary_null_comparison
     _clipRRect(rrect, doAntiAlias);
   }
 
   void _clipRRect(ui.RRect rrect, bool doAntiAlias) {
-    _canvas.clipRRect(rrect, doAntiAlias);
+    _canvas!.clipRRect(rrect, doAntiAlias);
   }
 
   @override
   void clipPath(ui.Path path, {bool doAntiAlias = true}) {
+    // ignore: unnecessary_null_comparison
     assert(path != null); // path is checked on the engine side
-    assert(doAntiAlias != null);
+    assert(doAntiAlias != null); // ignore: unnecessary_null_comparison
     _clipPath(path, doAntiAlias);
   }
 
   void _clipPath(ui.Path path, bool doAntiAlias) {
-    _canvas.clipPath(path, doAntiAlias);
+    _canvas!.clipPath(path, doAntiAlias);
   }
 
   @override
   void drawColor(ui.Color color, ui.BlendMode blendMode) {
-    assert(color != null);
-    assert(blendMode != null);
+    assert(color != null); // ignore: unnecessary_null_comparison
+    assert(blendMode != null); // ignore: unnecessary_null_comparison
     _drawColor(color, blendMode);
   }
 
   void _drawColor(ui.Color color, ui.BlendMode blendMode) {
-    _canvas.drawColor(color, blendMode);
+    _canvas!.drawColor(color, blendMode);
   }
 
   @override
   void drawLine(ui.Offset p1, ui.Offset p2, ui.Paint paint) {
     assert(_offsetIsValid(p1));
     assert(_offsetIsValid(p2));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawLine(p1, p2, paint);
   }
 
   void _drawLine(ui.Offset p1, ui.Offset p2, ui.Paint paint) {
-    _canvas.drawLine(p1, p2, paint);
+    _canvas!.drawLine(p1, p2, paint as SkPaint);
   }
 
   @override
   void drawPaint(ui.Paint paint) {
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawPaint(paint);
   }
 
   void _drawPaint(ui.Paint paint) {
-    _canvas.drawPaint(paint);
+    _canvas!.drawPaint(paint as SkPaint);
   }
 
   @override
   void drawRect(ui.Rect rect, ui.Paint paint) {
     assert(rectIsValid(rect));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawRect(rect, paint);
   }
 
   void _drawRect(ui.Rect rect, ui.Paint paint) {
-    _canvas.drawRect(rect, paint);
+    _canvas!.drawRect(rect, paint as SkPaint);
   }
 
   @override
   void drawRRect(ui.RRect rrect, ui.Paint paint) {
     assert(rrectIsValid(rrect));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawRRect(rrect, paint);
   }
 
   void _drawRRect(ui.RRect rrect, ui.Paint paint) {
-    _canvas.drawRRect(rrect, paint);
+    _canvas!.drawRRect(rrect, paint as SkPaint);
   }
 
   @override
   void drawDRRect(ui.RRect outer, ui.RRect inner, ui.Paint paint) {
     assert(rrectIsValid(outer));
     assert(rrectIsValid(inner));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawDRRect(outer, inner, paint);
   }
 
   void _drawDRRect(ui.RRect outer, ui.RRect inner, ui.Paint paint) {
-    _canvas.drawDRRect(outer, inner, paint);
+    _canvas!.drawDRRect(outer, inner, paint as SkPaint);
   }
 
   @override
   void drawOval(ui.Rect rect, ui.Paint paint) {
     assert(rectIsValid(rect));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawOval(rect, paint);
   }
 
   void _drawOval(ui.Rect rect, ui.Paint paint) {
-    _canvas.drawOval(rect, paint);
+    _canvas!.drawOval(rect, paint as SkPaint);
   }
 
   @override
   void drawCircle(ui.Offset c, double radius, ui.Paint paint) {
     assert(_offsetIsValid(c));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawCircle(c, radius, paint);
   }
 
   void _drawCircle(ui.Offset c, double radius, ui.Paint paint) {
-    _canvas.drawCircle(c, radius, paint);
+    _canvas!.drawCircle(c, radius, paint as SkPaint);
   }
 
   @override
   void drawArc(ui.Rect rect, double startAngle, double sweepAngle,
       bool useCenter, ui.Paint paint) {
     assert(rectIsValid(rect));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawArc(rect, startAngle, sweepAngle, useCenter, paint);
   }
 
   void _drawArc(ui.Rect rect, double startAngle, double sweepAngle,
       bool useCenter, ui.Paint paint) {
-    _canvas.drawArc(rect, startAngle, sweepAngle, useCenter, paint);
+    _canvas!.drawArc(rect, startAngle, sweepAngle, useCenter, paint as SkPaint);
   }
 
   @override
   void drawPath(ui.Path path, ui.Paint paint) {
+    // ignore: unnecessary_null_comparison
     assert(path != null); // path is checked on the engine side
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawPath(path, paint);
   }
 
   void _drawPath(ui.Path path, ui.Paint paint) {
-    _canvas.drawPath(path, paint);
+    _canvas!.drawPath(path, paint as SkPaint);
   }
 
   @override
   void drawImage(ui.Image image, ui.Offset p, ui.Paint paint) {
+    // ignore: unnecessary_null_comparison
     assert(image != null); // image is checked on the engine side
     assert(_offsetIsValid(p));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawImage(image, p, paint);
   }
 
   void _drawImage(ui.Image image, ui.Offset p, ui.Paint paint) {
-    _canvas.drawImage(image, p, paint);
+    _canvas!.drawImage(image, p, paint as SkPaint);
   }
 
   @override
   void drawImageRect(ui.Image image, ui.Rect src, ui.Rect dst, ui.Paint paint) {
+    // ignore: unnecessary_null_comparison
     assert(image != null); // image is checked on the engine side
     assert(rectIsValid(src));
     assert(rectIsValid(dst));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawImageRect(image, src, dst, paint);
   }
 
   void _drawImageRect(
       ui.Image image, ui.Rect src, ui.Rect dst, ui.Paint paint) {
-    _canvas.drawImageRect(image, src, dst, paint);
+    _canvas!.drawImageRect(image, src, dst, paint as SkPaint);
   }
 
   @override
   void drawImageNine(
       ui.Image image, ui.Rect center, ui.Rect dst, ui.Paint paint) {
+    // ignore: unnecessary_null_comparison
     assert(image != null); // image is checked on the engine side
     assert(rectIsValid(center));
     assert(rectIsValid(dst));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawImageNine(image, center, dst, paint);
   }
 
   void _drawImageNine(
       ui.Image image, ui.Rect center, ui.Rect dst, ui.Paint paint) {
-    _canvas.drawImageNine(image, center, dst, paint);
+    _canvas!.drawImageNine(image, center, dst, paint as SkPaint);
   }
 
   @override
   void drawPicture(ui.Picture picture) {
+    // ignore: unnecessary_null_comparison
     assert(picture != null); // picture is checked on the engine side
     _drawPicture(picture);
   }
 
   void _drawPicture(ui.Picture picture) {
-    _canvas.drawPicture(picture);
+    _canvas!.drawPicture(picture);
   }
 
   @override
   void drawParagraph(ui.Paragraph paragraph, ui.Offset offset) {
-    assert(paragraph != null);
+    assert(paragraph != null); // ignore: unnecessary_null_comparison
     assert(_offsetIsValid(offset));
     _drawParagraph(paragraph, offset);
   }
 
   void _drawParagraph(ui.Paragraph paragraph, ui.Offset offset) {
-    _canvas.drawParagraph(paragraph, offset);
+    _canvas!.drawParagraph(paragraph, offset);
   }
 
   @override
   void drawPoints(
       ui.PointMode pointMode, List<ui.Offset> points, ui.Paint paint) {
-    assert(pointMode != null);
-    assert(points != null);
-    assert(paint != null);
+    assert(pointMode != null); // ignore: unnecessary_null_comparison
+    assert(points != null); // ignore: unnecessary_null_comparison
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawPoints(paint, pointMode, encodePointList(points));
   }
 
   @override
   void drawRawPoints(
       ui.PointMode pointMode, Float32List points, ui.Paint paint) {
-    assert(pointMode != null);
-    assert(points != null);
-    assert(paint != null);
+    assert(pointMode != null); // ignore: unnecessary_null_comparison
+    assert(points != null); // ignore: unnecessary_null_comparison
+    assert(paint != null); // ignore: unnecessary_null_comparison
     if (points.length % 2 != 0) {
       throw ArgumentError('"points" must have an even number of values.');
     }
@@ -323,22 +329,23 @@ class CanvasKitCanvas implements ui.Canvas {
   }
 
   void _drawPoints(
-      ui.Paint paint, ui.PointMode pointMode, List<List<double>> points) {
-    _canvas.drawPoints(paint, pointMode, points);
+      ui.Paint paint, ui.PointMode pointMode, List<List<double>>? points) {
+    _canvas!.drawPoints(paint as SkPaint, pointMode, points as js.JsArray<js.JsArray<double>>?);
   }
 
   @override
   void drawVertices(
       ui.Vertices vertices, ui.BlendMode blendMode, ui.Paint paint) {
+    // ignore: unnecessary_null_comparison
     assert(vertices != null); // vertices is checked on the engine side
-    assert(paint != null);
-    assert(blendMode != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
+    assert(blendMode != null); // ignore: unnecessary_null_comparison
     _drawVertices(vertices, blendMode, paint);
   }
 
   void _drawVertices(
       ui.Vertices vertices, ui.BlendMode blendMode, ui.Paint paint) {
-    _canvas.drawVertices(vertices, blendMode, paint);
+    _canvas!.drawVertices(vertices, blendMode, paint as SkPaint);
   }
 
   @override
@@ -348,14 +355,15 @@ class CanvasKitCanvas implements ui.Canvas {
       List<ui.Rect> rects,
       List<ui.Color> colors,
       ui.BlendMode blendMode,
-      ui.Rect cullRect,
+      ui.Rect? cullRect,
       ui.Paint paint) {
+    // ignore: unnecessary_null_comparison
     assert(atlas != null); // atlas is checked on the engine side
-    assert(transforms != null);
-    assert(rects != null);
-    assert(colors != null);
-    assert(blendMode != null);
-    assert(paint != null);
+    assert(transforms != null); // ignore: unnecessary_null_comparison
+    assert(rects != null); // ignore: unnecessary_null_comparison
+    assert(colors != null); // ignore: unnecessary_null_comparison
+    assert(blendMode != null); // ignore: unnecessary_null_comparison
+    assert(paint != null); // ignore: unnecessary_null_comparison
 
     final int rectCount = rects.length;
     if (transforms.length != rectCount) {
@@ -387,7 +395,7 @@ class CanvasKitCanvas implements ui.Canvas {
       rectBuffer[index3] = rect.bottom;
     }
 
-    final js.JsArray<Float32List> colorBuffer =
+    final js.JsArray<Float32List>? colorBuffer =
         colors.isEmpty ? null : makeColorList(colors);
 
     _drawAtlas(
@@ -401,14 +409,15 @@ class CanvasKitCanvas implements ui.Canvas {
       Float32List rects,
       Int32List colors,
       ui.BlendMode blendMode,
-      ui.Rect cullRect,
+      ui.Rect? cullRect,
       ui.Paint paint) {
+    // ignore: unnecessary_null_comparison
     assert(atlas != null); // atlas is checked on the engine side
-    assert(rstTransforms != null);
-    assert(rects != null);
-    assert(colors != null);
-    assert(blendMode != null);
-    assert(paint != null);
+    assert(rstTransforms != null); // ignore: unnecessary_null_comparison
+    assert(rects != null); // ignore: unnecessary_null_comparison
+    assert(colors != null); // ignore: unnecessary_null_comparison
+    assert(blendMode != null); // ignore: unnecessary_null_comparison
+    assert(paint != null); // ignore: unnecessary_null_comparison
 
     final int rectCount = rects.length;
     if (rstTransforms.length != rectCount)
@@ -416,7 +425,7 @@ class CanvasKitCanvas implements ui.Canvas {
     if (rectCount % 4 != 0)
       throw ArgumentError(
           '"rstTransforms" and "rects" lengths must be a multiple of four.');
-    if (colors != null && colors.length * 4 != rectCount)
+    if (colors.length * 4 != rectCount)
       throw ArgumentError(
           'If non-null, "colors" length must be one fourth the length of "rstTransforms" and "rects".');
 
@@ -430,23 +439,24 @@ class CanvasKitCanvas implements ui.Canvas {
     ui.Image atlas,
     Float32List rstTransforms,
     Float32List rects,
-    js.JsArray<Float32List> colors,
+    js.JsArray<Float32List>? colors,
     ui.BlendMode blendMode,
   ) {
-    _canvas.drawAtlasRaw(paint, atlas, rstTransforms, rects, colors, blendMode);
+    _canvas!.drawAtlasRaw(paint as SkPaint, atlas, rstTransforms, rects, colors, blendMode);
   }
 
   @override
   void drawShadow(ui.Path path, ui.Color color, double elevation,
       bool transparentOccluder) {
+    // ignore: unnecessary_null_comparison
     assert(path != null); // path is checked on the engine side
-    assert(color != null);
-    assert(transparentOccluder != null);
+    assert(color != null); // ignore: unnecessary_null_comparison
+    assert(transparentOccluder != null); // ignore: unnecessary_null_comparison
     _drawShadow(path, color, elevation, transparentOccluder);
   }
 
   void _drawShadow(ui.Path path, ui.Color color, double elevation,
       bool transparentOccluder) {
-    _canvas.drawShadow(path, color, elevation, transparentOccluder);
+    _canvas!.drawShadow(path, color, elevation, transparentOccluder);
   }
 }
diff --git a/lib/web_ui/lib/src/engine/compositor/color_filter.dart b/lib/web_ui/lib/src/engine/compositor/color_filter.dart
index f3dae063f3075..f788bd73015fe 100644
--- a/lib/web_ui/lib/src/engine/compositor/color_filter.dart
+++ b/lib/web_ui/lib/src/engine/compositor/color_filter.dart
@@ -2,15 +2,15 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A [ui.ColorFilter] backed by Skia's [SkColorFilter].
 class SkColorFilter {
-  js.JsObject skColorFilter;
+  js.JsObject? skColorFilter;
 
   SkColorFilter.mode(EngineColorFilter filter) {
-    setSharedSkColor1(filter._color);
+    setSharedSkColor1(filter._color!);
     skColorFilter =
         canvasKit['SkColorFilter'].callMethod('MakeBlend', <dynamic>[
       sharedSkColor1,
@@ -23,7 +23,7 @@ class SkColorFilter {
     final js.JsArray<double> colorMatrix = js.JsArray<double>();
     colorMatrix.length = 20;
     for (int i = 0; i < 20; i++) {
-      colorMatrix[i] = filter._matrix[i];
+      colorMatrix[i] = filter._matrix![i];
     }
     skColorFilter = canvasKit['SkColorFilter']
         .callMethod('MakeMatrix', <js.JsArray>[colorMatrix]);
diff --git a/lib/web_ui/lib/src/engine/compositor/embedded_views.dart b/lib/web_ui/lib/src/engine/compositor/embedded_views.dart
index d5a76e55a3c36..4725a8dce9241 100644
--- a/lib/web_ui/lib/src/engine/compositor/embedded_views.dart
+++ b/lib/web_ui/lib/src/engine/compositor/embedded_views.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// This composites HTML views into the [ui.Scene].
@@ -25,10 +25,10 @@ class HtmlViewEmbedder {
       <int, EmbeddedViewParams>{};
 
   /// The HTML element associated with the given view id.
-  final Map<int, html.Element> _views = <int, html.Element>{};
+  final Map<int?, html.Element> _views = <int?, html.Element>{};
 
   /// The root view in the stack of mutator elements for the view id.
-  final Map<int, html.Element> _rootViews = <int, html.Element>{};
+  final Map<int?, html.Element?> _rootViews = <int?, html.Element?>{};
 
   /// The overlay for the view id.
   final Map<int, Overlay> _overlays = <int, Overlay>{};
@@ -37,7 +37,7 @@ class HtmlViewEmbedder {
   final Set<int> _viewsToRecomposite = <int>{};
 
   /// The views that need to be disposed of on the next frame.
-  final Set<int> _viewsToDispose = <int>{};
+  final Set<int?> _viewsToDispose = <int?>{};
 
   /// The list of view ids that should be composited, in order.
   List<int> _compositionOrder = <int>[];
@@ -49,7 +49,7 @@ class HtmlViewEmbedder {
   Map<int, int> _clipCount = <int, int>{};
 
   /// The size of the frame, in physical pixels.
-  ui.Size _frameSize;
+  ui.Size _frameSize = _computeFrameSize();
 
   void set frameSize(ui.Size size) {
     if (_frameSize == size) {
@@ -60,8 +60,8 @@ class HtmlViewEmbedder {
   }
 
   void handlePlatformViewCall(
-    ByteData data,
-    ui.PlatformMessageResponseCallback callback,
+    ByteData? data,
+    ui.PlatformMessageResponseCallback? callback,
   ) {
     const MethodCodec codec = StandardMethodCodec();
     final MethodCall decoded = codec.decodeMethodCall(data);
@@ -71,21 +71,21 @@ class HtmlViewEmbedder {
         _create(decoded, callback);
         return;
       case 'dispose':
-        _dispose(decoded, callback);
+        _dispose(decoded, callback!);
         return;
     }
-    callback(null);
+    callback!(null);
   }
 
   void _create(
-      MethodCall methodCall, ui.PlatformMessageResponseCallback callback) {
+      MethodCall methodCall, ui.PlatformMessageResponseCallback? callback) {
     final Map<dynamic, dynamic> args = methodCall.arguments;
-    final int viewId = args['id'];
-    final String viewType = args['viewType'];
+    final int? viewId = args['id'];
+    final String? viewType = args['viewType'];
     const MethodCodec codec = StandardMethodCodec();
 
     if (_views[viewId] != null) {
-      callback(codec.encodeErrorEnvelope(
+      callback!(codec.encodeErrorEnvelope(
         code: 'recreating_view',
         message: 'trying to create an already created view',
         details: 'view id: $viewId',
@@ -93,10 +93,10 @@ class HtmlViewEmbedder {
       return;
     }
 
-    final ui.PlatformViewFactory factory =
+    final ui.PlatformViewFactory? factory =
         ui.platformViewRegistry.registeredFactories[viewType];
     if (factory == null) {
-      callback(codec.encodeErrorEnvelope(
+      callback!(codec.encodeErrorEnvelope(
         code: 'unregistered_view_type',
         message: 'trying to create a view with an unregistered type',
         details: 'unregistered view type: $viewType',
@@ -105,17 +105,17 @@ class HtmlViewEmbedder {
     }
 
     // TODO(het): Support creation parameters.
-    html.Element embeddedView = factory(viewId);
+    html.Element embeddedView = factory(viewId!);
     _views[viewId] = embeddedView;
 
     _rootViews[viewId] = embeddedView;
 
-    callback(codec.encodeSuccessEnvelope(null));
+    callback!(codec.encodeSuccessEnvelope(null));
   }
 
   void _dispose(
       MethodCall methodCall, ui.PlatformMessageResponseCallback callback) {
-    int viewId = methodCall.arguments;
+    int? viewId = methodCall.arguments;
     const MethodCodec codec = StandardMethodCodec();
     if (!_views.containsKey(viewId)) {
       callback(codec.encodeErrorEnvelope(
@@ -128,11 +128,11 @@ class HtmlViewEmbedder {
     callback(codec.encodeSuccessEnvelope(null));
   }
 
-  List<SkCanvas> getCurrentCanvases() {
-    final List<SkCanvas> canvases = <SkCanvas>[];
+  List<SkCanvas?> getCurrentCanvases() {
+    final List<SkCanvas?> canvases = <SkCanvas?>[];
     for (int i = 0; i < _compositionOrder.length; i++) {
       final int viewId = _compositionOrder[i];
-      canvases.add(_pictureRecorders[viewId].recordingCanvas);
+      canvases.add(_pictureRecorders[viewId]!.recordingCanvas);
     }
     return canvases;
   }
@@ -140,7 +140,7 @@ class HtmlViewEmbedder {
   void prerollCompositeEmbeddedView(int viewId, EmbeddedViewParams params) {
     final pictureRecorder = SkPictureRecorder();
     pictureRecorder.beginRecording(ui.Offset.zero & _frameSize);
-    pictureRecorder.recordingCanvas.clear(ui.Color(0x00000000));
+    pictureRecorder.recordingCanvas!.clear(ui.Color(0x00000000));
     _pictureRecorders[viewId] = pictureRecorder;
     _compositionOrder.add(viewId);
 
@@ -152,28 +152,28 @@ class HtmlViewEmbedder {
     _viewsToRecomposite.add(viewId);
   }
 
-  SkCanvas compositeEmbeddedView(int viewId) {
+  SkCanvas? compositeEmbeddedView(int viewId) {
     // Do nothing if this view doesn't need to be composited.
     if (!_viewsToRecomposite.contains(viewId)) {
-      return _pictureRecorders[viewId].recordingCanvas;
+      return _pictureRecorders[viewId]!.recordingCanvas;
     }
-    _compositeWithParams(viewId, _currentCompositionParams[viewId]);
+    _compositeWithParams(viewId, _currentCompositionParams[viewId]!);
     _viewsToRecomposite.remove(viewId);
-    return _pictureRecorders[viewId].recordingCanvas;
+    return _pictureRecorders[viewId]!.recordingCanvas;
   }
 
   void _compositeWithParams(int viewId, EmbeddedViewParams params) {
-    final html.Element platformView = _views[viewId];
+    final html.Element platformView = _views[viewId]!;
     platformView.style.width = '${params.size.width}px';
     platformView.style.height = '${params.size.height}px';
     platformView.style.position = 'absolute';
 
     final int currentClippingCount = _countClips(params.mutators);
-    final int previousClippingCount = _clipCount[viewId];
+    final int? previousClippingCount = _clipCount[viewId];
     if (currentClippingCount != previousClippingCount) {
       _clipCount[viewId] = currentClippingCount;
-      html.Element oldPlatformViewRoot = _rootViews[viewId];
-      html.Element newPlatformViewRoot = _reconstructClipViewsChain(
+      html.Element oldPlatformViewRoot = _rootViews[viewId]!;
+      html.Element? newPlatformViewRoot = _reconstructClipViewsChain(
         currentClippingCount,
         platformView,
         oldPlatformViewRoot,
@@ -193,35 +193,35 @@ class HtmlViewEmbedder {
     return clipCount;
   }
 
-  html.Element _reconstructClipViewsChain(
+  html.Element? _reconstructClipViewsChain(
     int numClips,
     html.Element platformView,
     html.Element headClipView,
   ) {
     int indexInFlutterView = -1;
     if (headClipView.parent != null) {
-      indexInFlutterView = skiaSceneHost.children.indexOf(headClipView);
+      indexInFlutterView = skiaSceneHost!.children.indexOf(headClipView);
       headClipView.remove();
     }
-    html.Element head = platformView;
+    html.Element? head = platformView;
     int clipIndex = 0;
     // Re-use as much existing clip views as needed.
     while (head != headClipView && clipIndex < numClips) {
-      head = head.parent;
+      head = head!.parent;
       clipIndex++;
     }
     // If there weren't enough existing clip views, add more.
     while (clipIndex < numClips) {
       html.Element clippingView = html.Element.tag('flt-clip');
-      clippingView.append(head);
+      clippingView.append(head!);
       head = clippingView;
       clipIndex++;
     }
-    head.remove();
+    head!.remove();
 
     // If the chain was previously attached, attach it to the same position.
     if (indexInFlutterView > -1) {
-      skiaSceneHost.children.insert(indexInFlutterView, head);
+      skiaSceneHost!.children.insert(indexInFlutterView, head);
     }
     return head;
   }
@@ -235,27 +235,27 @@ class HtmlViewEmbedder {
     for (final Mutator mutator in mutators) {
       switch (mutator.type) {
         case MutatorType.transform:
-          headTransform.multiply(mutator.matrix);
+          headTransform.multiply(mutator.matrix!);
           head.style.transform =
               float64ListToCssTransform(headTransform.storage);
           break;
         case MutatorType.clipRect:
         case MutatorType.clipRRect:
         case MutatorType.clipPath:
-          html.Element clipView = head.parent;
+          html.Element clipView = head.parent!;
           clipView.style.clip = '';
           clipView.style.clipPath = '';
           headTransform = Matrix4.identity();
           clipView.style.transform = '';
           if (mutator.rect != null) {
-            final ui.Rect rect = mutator.rect;
+            final ui.Rect rect = mutator.rect!;
             clipView.style.clip = 'rect(${rect.top}px, ${rect.right}px, '
                 '${rect.bottom}px, ${rect.left}px)';
           } else if (mutator.rrect != null) {
             final SkPath path = SkPath();
-            path.addRRect(mutator.rrect);
+            path.addRRect(mutator.rrect!);
             _ensureSvgPathDefs();
-            html.Element pathDefs = _svgPathDefs.querySelector('#sk_path_defs');
+            html.Element pathDefs = _svgPathDefs!.querySelector('#sk_path_defs')!;
             _clipPathCount += 1;
             html.Element newClipPath =
                 html.Element.html('<clipPath id="svgClip$_clipPathCount">'
@@ -264,9 +264,9 @@ class HtmlViewEmbedder {
             pathDefs.append(newClipPath);
             clipView.style.clipPath = 'url(#svgClip$_clipPathCount)';
           } else if (mutator.path != null) {
-            final SkPath path = mutator.path;
+            final SkPath path = mutator.path as SkPath;
             _ensureSvgPathDefs();
-            html.Element pathDefs = _svgPathDefs.querySelector('#sk_path_defs');
+            html.Element pathDefs = _svgPathDefs!.querySelector('#sk_path_defs')!;
             _clipPathCount += 1;
             html.Element newClipPath =
                 html.Element.html('<clipPath id="svgClip$_clipPathCount">'
@@ -309,7 +309,7 @@ class HtmlViewEmbedder {
 
   int _clipPathCount = 0;
 
-  html.Element _svgPathDefs;
+  html.Element? _svgPathDefs;
 
   /// Ensures we add a container of SVG path defs to the DOM so they can
   /// be referred to in clip-path: url(#blah).
@@ -321,7 +321,7 @@ class HtmlViewEmbedder {
       '<svg width="0" height="0"><defs id="sk_path_defs"></defs></svg>',
       treeSanitizer: _NullTreeSanitizer(),
     );
-    skiaSceneHost.append(_svgPathDefs);
+    skiaSceneHost!.append(_svgPathDefs!);
   }
 
   void submitFrame() {
@@ -331,9 +331,9 @@ class HtmlViewEmbedder {
       int viewId = _compositionOrder[i];
       ensureOverlayInitialized(viewId);
       final SurfaceFrame frame =
-          _overlays[viewId].surface.acquireFrame(_frameSize);
+          _overlays[viewId]!.surface.acquireFrame(_frameSize);
       final SkCanvas canvas = frame.skiaCanvas;
-      canvas.drawPicture(_pictureRecorders[viewId].endRecording());
+      canvas.drawPicture(_pictureRecorders[viewId]!.endRecording());
       frame.submit();
     }
     _pictureRecorders.clear();
@@ -345,12 +345,12 @@ class HtmlViewEmbedder {
 
     for (int i = 0; i < _compositionOrder.length; i++) {
       int viewId = _compositionOrder[i];
-      html.Element platformViewRoot = _rootViews[viewId];
-      html.Element overlay = _overlays[viewId].surface.htmlElement;
+      html.Element platformViewRoot = _rootViews[viewId]!;
+      html.Element overlay = _overlays[viewId]!.surface.htmlElement!;
       platformViewRoot.remove();
-      skiaSceneHost.append(platformViewRoot);
+      skiaSceneHost!.append(platformViewRoot);
       overlay.remove();
-      skiaSceneHost.append(overlay);
+      skiaSceneHost!.append(overlay);
       _activeCompositionOrder.add(viewId);
     }
     _compositionOrder.clear();
@@ -361,13 +361,13 @@ class HtmlViewEmbedder {
       return;
     }
 
-    for (int viewId in _viewsToDispose) {
-      final html.Element rootView = _rootViews[viewId];
+    for (int? viewId in _viewsToDispose) {
+      final html.Element rootView = _rootViews[viewId]!;
       rootView.remove();
       _views.remove(viewId);
       _rootViews.remove(viewId);
       if (_overlays[viewId] != null) {
-        final Overlay overlay = _overlays[viewId];
+        final Overlay overlay = _overlays[viewId]!;
         overlay.surface.htmlElement?.remove();
       }
       _overlays.remove(viewId);
@@ -379,12 +379,12 @@ class HtmlViewEmbedder {
   }
 
   void ensureOverlayInitialized(int viewId) {
-    Overlay overlay = _overlays[viewId];
+    Overlay? overlay = _overlays[viewId];
     if (overlay != null) {
       return;
     }
-    Surface surface = Surface();
-    SkSurface skSurface = surface.acquireRenderSurface(_frameSize);
+    Surface surface = Surface(this);
+    SkSurface? skSurface = surface.acquireRenderSurface(_frameSize);
     _overlays[viewId] = Overlay(surface, skSurface);
   }
 }
@@ -435,11 +435,11 @@ class Mutator {
   );
 
   final MutatorType type;
-  final ui.Rect rect;
-  final ui.RRect rrect;
-  final ui.Path path;
-  final Matrix4 matrix;
-  final int alpha;
+  final ui.Rect? rect;
+  final ui.RRect? rrect;
+  final ui.Path? path;
+  final Matrix4? matrix;
+  final int? alpha;
 
   const Mutator.clipRect(ui.Rect rect)
       : this._(MutatorType.clipRect, rect, null, null, null, null);
@@ -457,7 +457,7 @@ class Mutator {
       type == MutatorType.clipRRect ||
       type == MutatorType.clipPath;
 
-  double get alphaFloat => alpha / 255.0;
+  double get alphaFloat => alpha! / 255.0;
 
   bool operator ==(dynamic other) {
     if (identical(this, other)) {
@@ -555,7 +555,7 @@ class MutatorsStack extends Iterable<Mutator> {
 /// Represents a surface overlaying a platform view.
 class Overlay {
   final Surface surface;
-  final SkSurface skSurface;
+  final SkSurface? skSurface;
 
   Overlay(this.surface, this.skSurface);
 }
diff --git a/lib/web_ui/lib/src/engine/compositor/fonts.dart b/lib/web_ui/lib/src/engine/compositor/fonts.dart
index 0d913b0de4a09..1bce459d2207e 100644
--- a/lib/web_ui/lib/src/engine/compositor/fonts.dart
+++ b/lib/web_ui/lib/src/engine/compositor/fonts.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 // This URL was found by using the Google Fonts Developer API to find the URL
@@ -16,11 +16,11 @@ const String _robotoUrl =
 /// Manages the fonts used in the Skia-based backend.
 class SkiaFontCollection {
   /// Fonts that have been registered but haven't been loaded yet.
-  final List<Future<_RegisteredFont>> _unloadedFonts =
+  final List<Future<_RegisteredFont?>> _unloadedFonts =
       <Future<_RegisteredFont>>[];
 
   /// Fonts which have been registered and loaded.
-  final List<_RegisteredFont> _registeredFonts = <_RegisteredFont>[];
+  final List<_RegisteredFont?> _registeredFonts = <_RegisteredFont?>[];
 
   /// A mapping from the name a font was registered with, to the family name
   /// embedded in the font's bytes (the font's "actual" name).
@@ -32,14 +32,14 @@ class SkiaFontCollection {
   /// must convert them to their actual family name when we pass them to Skia.
   final Map<String, String> fontFamilyOverrides = <String, String>{};
 
-  final Set<String> registeredFamilies = <String>{};
+  final Set<String?> registeredFamilies = <String?>{};
 
   Future<void> ensureFontsLoaded() async {
     await _loadFonts();
     _computeFontFamilyOverrides();
 
     final List<Uint8List> fontBuffers =
-        _registeredFonts.map<Uint8List>((f) => f.bytes).toList();
+        _registeredFonts.map<Uint8List>((f) => f!.bytes).toList();
 
     skFontMgr = canvasKit['SkFontMgr'].callMethod('FromData', fontBuffers);
   }
@@ -51,7 +51,7 @@ class SkiaFontCollection {
       return;
     }
 
-    final List<_RegisteredFont> loadedFonts = await Future.wait(_unloadedFonts);
+    final List<_RegisteredFont?> loadedFonts = await Future.wait(_unloadedFonts);
     _registeredFonts.addAll(loadedFonts.where((x) => x != null));
     _unloadedFonts.clear();
   }
@@ -59,8 +59,8 @@ class SkiaFontCollection {
   void _computeFontFamilyOverrides() {
     fontFamilyOverrides.clear();
 
-    for (_RegisteredFont font in _registeredFonts) {
-      if (fontFamilyOverrides.containsKey(font.flutterFamily)) {
+    for (_RegisteredFont? font in _registeredFonts) {
+      if (fontFamilyOverrides.containsKey(font!.flutterFamily)) {
         if (fontFamilyOverrides[font.flutterFamily] != font.actualFamily) {
           html.window.console.warn('Fonts in family ${font.flutterFamily} '
               'have different actual family names.');
@@ -74,8 +74,8 @@ class SkiaFontCollection {
     }
   }
 
-  Future<void> loadFontFromList(Uint8List list, {String fontFamily}) async {
-    String actualFamily = _readActualFamilyName(list);
+  Future<void> loadFontFromList(Uint8List list, {String? fontFamily}) async {
+    String? actualFamily = _readActualFamilyName(list);
 
     if (actualFamily == null) {
       if (fontFamily == null) {
@@ -111,20 +111,15 @@ class SkiaFontCollection {
       }
     }
 
-    if (byteData == null) {
-      throw AssertionError(
-          'There was a problem trying to load FontManifest.json');
-    }
-
-    final List<dynamic> fontManifest =
+    final List<dynamic>? fontManifest =
         json.decode(utf8.decode(byteData.buffer.asUint8List()));
     if (fontManifest == null) {
       throw AssertionError(
           'There was a problem trying to load FontManifest.json');
     }
 
-    for (Map<String, dynamic> fontFamily in fontManifest) {
-      final String family = fontFamily['family'];
+    for (Map<String, dynamic> fontFamily in fontManifest.cast<Map<String, dynamic>>()) {
+      final String? family = fontFamily['family'];
       final List<dynamic> fontAssets = fontFamily['fonts'];
 
       registeredFamilies.add(family);
@@ -146,10 +141,10 @@ class SkiaFontCollection {
     }
   }
 
-  Future<_RegisteredFont> _registerFont(String url, String family) async {
+  Future<_RegisteredFont?> _registerFont(String url, String? family) async {
     ByteBuffer buffer;
     try {
-      buffer = await html.window.fetch(url).then(_getArrayBuffer);
+      buffer = await html.window.fetch(url).then(_getArrayBuffer as FutureOr<ByteBuffer> Function(dynamic));
     } catch (e) {
       html.window.console.warn('Failed to load font $family at $url');
       html.window.console.warn(e);
@@ -157,7 +152,7 @@ class SkiaFontCollection {
     }
 
     final Uint8List bytes = buffer.asUint8List();
-    String actualFamily = _readActualFamilyName(bytes);
+    String? actualFamily = _readActualFamilyName(bytes);
 
     if (actualFamily == null) {
       html.window.console.warn('Failed to determine the actual name of the '
@@ -165,25 +160,25 @@ class SkiaFontCollection {
       actualFamily = family;
     }
 
-    return _RegisteredFont(bytes, family, actualFamily);
+    return _RegisteredFont(bytes, family!, actualFamily!);
   }
 
-  String _readActualFamilyName(Uint8List bytes) {
+  String? _readActualFamilyName(Uint8List bytes) {
     final js.JsObject tmpFontMgr =
         canvasKit['SkFontMgr'].callMethod('FromData', <Uint8List>[bytes]);
-    String actualFamily = tmpFontMgr.callMethod('getFamilyName', <int>[0]);
+    String? actualFamily = tmpFontMgr.callMethod('getFamilyName', <int>[0]);
     tmpFontMgr.callMethod('delete');
     return actualFamily;
   }
 
-  Future<ByteBuffer> _getArrayBuffer(dynamic fetchResult) {
+  Future<ByteBuffer>? _getArrayBuffer(dynamic fetchResult) {
     // TODO(yjbanov): fetchResult.arrayBuffer is a dynamic invocation. Clean it up.
     return fetchResult
         .arrayBuffer()
         .then<ByteBuffer>((dynamic x) => x as ByteBuffer);
   }
 
-  js.JsObject skFontMgr;
+  js.JsObject? skFontMgr;
 }
 
 /// Represents a font that has been registered.
@@ -198,7 +193,7 @@ class _RegisteredFont {
   final String actualFamily;
 
   _RegisteredFont(this.bytes, this.flutterFamily, this.actualFamily)
-      : assert(bytes != null),
-        assert(flutterFamily != null),
-        assert(actualFamily != null);
+      : assert(bytes != null), // ignore: unnecessary_null_comparison
+        assert(flutterFamily != null), // ignore: unnecessary_null_comparison
+        assert(actualFamily != null); // ignore: unnecessary_null_comparison
 }
diff --git a/lib/web_ui/lib/src/engine/compositor/image.dart b/lib/web_ui/lib/src/engine/compositor/image.dart
index 7816067269b85..a341aeb6c9c9a 100644
--- a/lib/web_ui/lib/src/engine/compositor/image.dart
+++ b/lib/web_ui/lib/src/engine/compositor/image.dart
@@ -2,13 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Instantiates a [ui.Codec] backed by an `SkImage` from Skia.
 void skiaInstantiateImageCodec(Uint8List list, Callback<ui.Codec> callback,
-    [int width, int height, int format, int rowBytes]) {
-  final js.JsObject skAnimatedImage =
+    [int? width, int? height, int? format, int? rowBytes]) {
+  final js.JsObject? skAnimatedImage =
       canvasKit.callMethod('MakeAnimatedImageFromEncoded', <Uint8List>[list]);
   final SkAnimatedImage animatedImage = SkAnimatedImage(skAnimatedImage);
   final SkAnimatedImageCodec codec = SkAnimatedImageCodec(animatedImage);
@@ -17,36 +17,36 @@ void skiaInstantiateImageCodec(Uint8List list, Callback<ui.Codec> callback,
 
 /// A wrapper for `SkAnimatedImage`.
 class SkAnimatedImage implements ui.Image {
-  final js.JsObject _skAnimatedImage;
+  final js.JsObject? _skAnimatedImage;
 
   SkAnimatedImage(this._skAnimatedImage);
 
   @override
   void dispose() {
-    _skAnimatedImage.callMethod('delete');
+    _skAnimatedImage!.callMethod('delete');
   }
 
-  int get frameCount => _skAnimatedImage.callMethod('getFrameCount');
+  int? get frameCount => _skAnimatedImage!.callMethod('getFrameCount');
 
   /// Decodes the next frame and returns the frame duration.
   Duration decodeNextFrame() {
-    final int durationMillis = _skAnimatedImage.callMethod('decodeNextFrame');
+    final int durationMillis = _skAnimatedImage!.callMethod('decodeNextFrame');
     return Duration(milliseconds: durationMillis);
   }
 
-  int get repetitionCount => _skAnimatedImage.callMethod('getRepetitionCount');
+  int? get repetitionCount => _skAnimatedImage!.callMethod('getRepetitionCount');
 
   SkImage get currentFrameAsImage {
-    final js.JsObject _currentFrame =
-        _skAnimatedImage.callMethod('getCurrentFrame');
+    final js.JsObject? _currentFrame =
+        _skAnimatedImage!.callMethod('getCurrentFrame');
     return SkImage(_currentFrame);
   }
 
   @override
-  int get width => _skAnimatedImage.callMethod('width');
+  int get width => _skAnimatedImage!.callMethod('width');
 
   @override
-  int get height => _skAnimatedImage.callMethod('height');
+  int get height => _skAnimatedImage!.callMethod('height');
 
   @override
   Future<ByteData> toByteData(
@@ -57,21 +57,21 @@ class SkAnimatedImage implements ui.Image {
 
 /// A [ui.Image] backed by an `SkImage` from Skia.
 class SkImage implements ui.Image {
-  js.JsObject skImage;
+  js.JsObject? skImage;
 
   SkImage(this.skImage);
 
   @override
   void dispose() {
-    skImage.callMethod('delete');
+    skImage!.callMethod('delete');
     skImage = null;
   }
 
   @override
-  int get width => skImage.callMethod('width');
+  int get width => skImage!.callMethod('width');
 
   @override
-  int get height => skImage.callMethod('height');
+  int get height => skImage!.callMethod('height');
 
   @override
   Future<ByteData> toByteData(
@@ -82,26 +82,26 @@ class SkImage implements ui.Image {
 
 /// A [Codec] that wraps an `SkAnimatedImage`.
 class SkAnimatedImageCodec implements ui.Codec {
-  SkAnimatedImage animatedImage;
+  SkAnimatedImage? animatedImage;
 
   SkAnimatedImageCodec(this.animatedImage);
 
   @override
   void dispose() {
-    animatedImage.dispose();
+    animatedImage!.dispose();
     animatedImage = null;
   }
 
   @override
-  int get frameCount => animatedImage.frameCount;
+  int get frameCount => animatedImage!.frameCount!;
 
   @override
-  int get repetitionCount => animatedImage.repetitionCount;
+  int get repetitionCount => animatedImage!.repetitionCount!;
 
   @override
   Future<ui.FrameInfo> getNextFrame() {
-    final Duration duration = animatedImage.decodeNextFrame();
-    final SkImage image = animatedImage.currentFrameAsImage;
+    final Duration duration = animatedImage!.decodeNextFrame();
+    final SkImage image = animatedImage!.currentFrameAsImage;
     return Future<ui.FrameInfo>.value(AnimatedImageFrameInfo(duration, image));
   }
 }
diff --git a/lib/web_ui/lib/src/engine/compositor/image_filter.dart b/lib/web_ui/lib/src/engine/compositor/image_filter.dart
index 8ea97082a2e10..79cc64abadf1d 100644
--- a/lib/web_ui/lib/src/engine/compositor/image_filter.dart
+++ b/lib/web_ui/lib/src/engine/compositor/image_filter.dart
@@ -2,14 +2,14 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// The CanvasKit implementation of [ui.ImageFilter].
 ///
 /// Currently only supports `blur`.
 class SkImageFilter implements ui.ImageFilter {
-  js.JsObject skImageFilter;
+  js.JsObject? skImageFilter;
 
   SkImageFilter.blur({double sigmaX = 0.0, double sigmaY = 0.0})
       : _sigmaX = sigmaX,
diff --git a/lib/web_ui/lib/src/engine/compositor/initialization.dart b/lib/web_ui/lib/src/engine/compositor/initialization.dart
index 1499ffb49bdbd..0052f5c9bfd5a 100644
--- a/lib/web_ui/lib/src/engine/compositor/initialization.dart
+++ b/lib/web_ui/lib/src/engine/compositor/initialization.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// EXPERIMENTAL: Enable the Skia-based rendering backend.
@@ -21,8 +21,8 @@ const String canvasKitBaseUrl = 'https://unpkg.com/canvaskit-wasm@0.16.2/bin/';
 /// This calls `CanvasKitInit` and assigns the global [canvasKit] object.
 Future<void> initializeSkia() {
   final Completer<void> canvasKitCompleter = Completer<void>();
-  StreamSubscription<html.Event> loadSubscription;
-  loadSubscription = domRenderer.canvasKitScript.onLoad.listen((_) {
+  late StreamSubscription<html.Event> loadSubscription;
+  loadSubscription = domRenderer.canvasKitScript!.onLoad.listen((_) {
     loadSubscription.cancel();
     final js.JsObject canvasKitInitArgs = js.JsObject.jsify(<String, dynamic>{
       'locateFile': (String file, String unusedBase) => canvasKitBaseUrl + file,
@@ -46,15 +46,16 @@ Future<void> initializeSkia() {
 /// The entrypoint into all CanvasKit functions and classes.
 ///
 /// This is created by [initializeSkia].
-js.JsObject canvasKit;
+late js.JsObject canvasKit;
 
 /// The Skia font collection.
-SkiaFontCollection skiaFontCollection;
+SkiaFontCollection get skiaFontCollection => _skiaFontCollection!;
+SkiaFontCollection? _skiaFontCollection;
 
 /// Initializes [skiaFontCollection].
 void ensureSkiaFontCollectionInitialized() {
-  skiaFontCollection ??= SkiaFontCollection();
+  _skiaFontCollection ??= SkiaFontCollection();
 }
 
 /// The scene host, where the root canvas and overlay canvases are added to.
-html.Element skiaSceneHost;
+html.Element? skiaSceneHost;
diff --git a/lib/web_ui/lib/src/engine/compositor/layer.dart b/lib/web_ui/lib/src/engine/compositor/layer.dart
index d30dc573a16b2..db70bbcab48a2 100644
--- a/lib/web_ui/lib/src/engine/compositor/layer.dart
+++ b/lib/web_ui/lib/src/engine/compositor/layer.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A layer to be composed into a scene.
@@ -11,7 +11,7 @@ part of engine;
 /// painting command.
 abstract class Layer implements ui.EngineLayer {
   /// The layer that contains us as a child.
-  ContainerLayer parent;
+  ContainerLayer? parent;
 
   /// An estimated rectangle that this layer will draw into.
   ui.Rect paintBounds = ui.Rect.zero;
@@ -33,10 +33,10 @@ abstract class Layer implements ui.EngineLayer {
 /// A context shared by all layers during the preroll pass.
 class PrerollContext {
   /// A raster cache. Used to register candidates for caching.
-  final RasterCache rasterCache;
+  final RasterCache? rasterCache;
 
   /// A compositor for embedded HTML views.
-  final HtmlViewEmbedder viewEmbedder;
+  final HtmlViewEmbedder? viewEmbedder;
 
   final MutatorsStack mutatorsStack = MutatorsStack();
 
@@ -51,13 +51,13 @@ class PaintContext {
   SkNWayCanvas internalNodesCanvas;
 
   /// The canvas for leaf nodes to paint to.
-  SkCanvas leafNodesCanvas;
+  SkCanvas? leafNodesCanvas;
 
   /// A raster cache potentially containing pre-rendered pictures.
-  final RasterCache rasterCache;
+  final RasterCache? rasterCache;
 
   /// A compositor for embedded HTML views.
-  final HtmlViewEmbedder viewEmbedder;
+  final HtmlViewEmbedder? viewEmbedder;
 
   PaintContext(
     this.internalNodesCanvas,
@@ -207,7 +207,7 @@ class ClipRectLayer extends ContainerLayer {
 class ClipRRectLayer extends ContainerLayer {
   /// The rounded rectangle used to clip child layers.
   final ui.RRect _clipRRect;
-  final ui.Clip _clipBehavior;
+  final ui.Clip? _clipBehavior;
 
   ClipRRectLayer(this._clipRRect, this._clipBehavior)
       : assert(_clipBehavior != ui.Clip.none);
@@ -379,19 +379,19 @@ class PictureLayer extends Layer {
 
   @override
   void preroll(PrerollContext prerollContext, Matrix4 matrix) {
-    paintBounds = picture.cullRect.shift(offset);
+    paintBounds = picture.cullRect!.shift(offset);
   }
 
   @override
   void paint(PaintContext paintContext) {
-    assert(picture != null);
+    assert(picture != null); // ignore: unnecessary_null_comparison
     assert(needsPainting);
 
-    paintContext.leafNodesCanvas.save();
-    paintContext.leafNodesCanvas.translate(offset.dx, offset.dy);
+    paintContext.leafNodesCanvas!.save();
+    paintContext.leafNodesCanvas!.translate(offset.dx, offset.dy);
 
-    paintContext.leafNodesCanvas.drawPicture(picture);
-    paintContext.leafNodesCanvas.restore();
+    paintContext.leafNodesCanvas!.drawPicture(picture);
+    paintContext.leafNodesCanvas!.restore();
   }
 }
 
@@ -401,10 +401,10 @@ class PictureLayer extends Layer {
 /// on the given elevation.
 class PhysicalShapeLayer extends ContainerLayer
     implements ui.PhysicalShapeEngineLayer {
-  final double _elevation;
-  final ui.Color _color;
-  final ui.Color _shadowColor;
-  final ui.Path _path;
+  final double? _elevation;
+  final ui.Color? _color;
+  final ui.Color? _shadowColor;
+  final ui.Path? _path;
   final ui.Clip _clipBehavior;
 
   PhysicalShapeLayer(
@@ -419,7 +419,7 @@ class PhysicalShapeLayer extends ContainerLayer
   void preroll(PrerollContext prerollContext, Matrix4 matrix) {
     prerollChildren(prerollContext, matrix);
 
-    paintBounds = _path.getBounds();
+    paintBounds = _path!.getBounds();
     if (_elevation == 0.0) {
       // No need to extend the paint bounds if there is no shadow.
       return;
@@ -479,16 +479,16 @@ class PhysicalShapeLayer extends ContainerLayer
     assert(needsPainting);
 
     if (_elevation != 0) {
-      drawShadow(paintContext.leafNodesCanvas, _path, _shadowColor, _elevation,
-          _color.alpha != 0xff);
+      drawShadow(paintContext.leafNodesCanvas!, _path!, _shadowColor!, _elevation!,
+          _color!.alpha != 0xff);
     }
 
-    final ui.Paint paint = ui.Paint()..color = _color;
+    final ui.Paint paint = ui.Paint()..color = _color!;
     if (_clipBehavior != ui.Clip.antiAliasWithSaveLayer) {
-      paintContext.leafNodesCanvas.drawPath(_path, paint);
+      paintContext.leafNodesCanvas!.drawPath(_path!, paint as SkPaint);
     }
 
-    final int saveCount = paintContext.internalNodesCanvas.save();
+    final int? saveCount = paintContext.internalNodesCanvas.save();
     switch (_clipBehavior) {
       case ui.Clip.hardEdge:
         paintContext.internalNodesCanvas.clipPath(_path, false);
@@ -509,7 +509,7 @@ class PhysicalShapeLayer extends ContainerLayer
       // (https://github.com/flutter/flutter/issues/18057#issue-328003931)
       // using saveLayer, we have to call drawPaint instead of drawPath as
       // anti-aliased drawPath will always have such artifacts.
-      paintContext.leafNodesCanvas.drawPaint(paint);
+      paintContext.leafNodesCanvas!.drawPaint(paint as SkPaint);
     }
 
     paintChildren(paintContext);
@@ -539,7 +539,7 @@ class PlatformViewLayer extends Layer {
   @override
   void preroll(PrerollContext context, Matrix4 matrix) {
     paintBounds = ui.Rect.fromLTWH(offset.dx, offset.dy, width, height);
-    context.viewEmbedder.prerollCompositeEmbeddedView(
+    context.viewEmbedder!.prerollCompositeEmbeddedView(
       viewId,
       EmbeddedViewParams(
         offset,
@@ -551,7 +551,7 @@ class PlatformViewLayer extends Layer {
 
   @override
   void paint(PaintContext context) {
-    SkCanvas canvas = context.viewEmbedder.compositeEmbeddedView(viewId);
+    SkCanvas? canvas = context.viewEmbedder!.compositeEmbeddedView(viewId);
     context.leafNodesCanvas = canvas;
   }
 }
diff --git a/lib/web_ui/lib/src/engine/compositor/layer_scene_builder.dart b/lib/web_ui/lib/src/engine/compositor/layer_scene_builder.dart
index 64c0ac8802eaf..e7377b915a68b 100644
--- a/lib/web_ui/lib/src/engine/compositor/layer_scene_builder.dart
+++ b/lib/web_ui/lib/src/engine/compositor/layer_scene_builder.dart
@@ -2,13 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 class LayerScene implements ui.Scene {
   final LayerTree layerTree;
 
-  LayerScene(Layer rootLayer) : layerTree = LayerTree() {
+  LayerScene(Layer? rootLayer) : layerTree = LayerTree() {
     layerTree.rootLayer = rootLayer;
   }
 
@@ -16,21 +16,21 @@ class LayerScene implements ui.Scene {
   void dispose() {}
 
   @override
-  Future<ui.Image> toImage(int width, int height) => null;
-
-  html.Element get webOnlyRootElement => null;
+  Future<ui.Image> toImage(int width, int height) {
+    throw UnsupportedError('LayerScene.toImage not implemented.');
+  }
 }
 
 class LayerSceneBuilder implements ui.SceneBuilder {
-  Layer rootLayer;
-  ContainerLayer currentLayer;
+  Layer? rootLayer;
+  ContainerLayer? currentLayer;
 
   @override
   void addChildScene({
     ui.Offset offset = ui.Offset.zero,
     double width = 0.0,
     double height = 0.0,
-    ui.SceneHost sceneHost,
+    ui.SceneHost? sceneHost,
     bool hitTestable = true,
   }) {
     throw UnimplementedError();
@@ -49,7 +49,7 @@ class LayerSceneBuilder implements ui.SceneBuilder {
     bool isComplexHint = false,
     bool willChangeHint = false,
   }) {
-    currentLayer.add(PictureLayer(picture, offset, isComplexHint, willChangeHint));
+    currentLayer!.add(PictureLayer(picture as SkPicture, offset, isComplexHint, willChangeHint));
   }
 
   @override
@@ -57,7 +57,7 @@ class LayerSceneBuilder implements ui.SceneBuilder {
     if (currentLayer == null) {
       return;
     }
-    currentLayer.add(retainedLayer);
+    currentLayer!.add(retainedLayer as Layer);
   }
 
   @override
@@ -78,9 +78,9 @@ class LayerSceneBuilder implements ui.SceneBuilder {
     ui.Offset offset = ui.Offset.zero,
     double width = 0.0,
     double height = 0.0,
-    Object webOnlyPaintedBy,
+    Object? webOnlyPaintedBy,
   }) {
-    currentLayer.add(PlatformViewLayer(viewId, offset, width, height));
+    currentLayer!.add(PlatformViewLayer(viewId, offset, width, height));
   }
 
   @override
@@ -93,43 +93,43 @@ class LayerSceneBuilder implements ui.SceneBuilder {
     if (currentLayer == null) {
       return;
     }
-    currentLayer = currentLayer.parent;
+    currentLayer = currentLayer!.parent;
   }
 
   @override
-  ui.BackdropFilterEngineLayer pushBackdropFilter(
+  ui.BackdropFilterEngineLayer? pushBackdropFilter(
     ui.ImageFilter filter, {
-    ui.EngineLayer oldLayer,
+    ui.EngineLayer? oldLayer,
   }) {
     pushLayer(BackdropFilterLayer(filter));
     return null;
   }
 
   @override
-  ui.ClipPathEngineLayer pushClipPath(
+  ui.ClipPathEngineLayer? pushClipPath(
     ui.Path path, {
     ui.Clip clipBehavior = ui.Clip.antiAlias,
-    ui.EngineLayer oldLayer,
+    ui.EngineLayer? oldLayer,
   }) {
     pushLayer(ClipPathLayer(path, clipBehavior));
     return null;
   }
 
   @override
-  ui.ClipRRectEngineLayer pushClipRRect(
+  ui.ClipRRectEngineLayer? pushClipRRect(
     ui.RRect rrect, {
-    ui.Clip clipBehavior,
-    ui.EngineLayer oldLayer,
+    ui.Clip? clipBehavior,
+    ui.EngineLayer? oldLayer,
   }) {
     pushLayer(ClipRRectLayer(rrect, clipBehavior));
     return null;
   }
 
   @override
-  ui.ClipRectEngineLayer pushClipRect(
+  ui.ClipRectEngineLayer? pushClipRect(
     ui.Rect rect, {
     ui.Clip clipBehavior = ui.Clip.antiAlias,
-    ui.EngineLayer oldLayer,
+    ui.EngineLayer? oldLayer,
   }) {
     pushLayer(ClipRectLayer(rect, clipBehavior));
     return null;
@@ -138,17 +138,17 @@ class LayerSceneBuilder implements ui.SceneBuilder {
   @override
   ui.ColorFilterEngineLayer pushColorFilter(
     ui.ColorFilter filter, {
-    ui.ColorFilterEngineLayer oldLayer,
+    ui.ColorFilterEngineLayer? oldLayer,
   }) {
-    assert(filter != null);
+    assert(filter != null); // ignore: unnecessary_null_comparison
     throw UnimplementedError();
   }
 
-  ui.ImageFilterEngineLayer pushImageFilter(
+  ui.ImageFilterEngineLayer? pushImageFilter(
     ui.ImageFilter filter, {
-    ui.ImageFilterEngineLayer oldLayer,
+    ui.ImageFilterEngineLayer? oldLayer,
   }) {
-    assert(filter != null);
+    assert(filter != null); // ignore: unnecessary_null_comparison
     pushLayer(ImageFilterLayer(filter));
     return null;
   }
@@ -157,7 +157,7 @@ class LayerSceneBuilder implements ui.SceneBuilder {
   ui.OffsetEngineLayer pushOffset(
     double dx,
     double dy, {
-    ui.EngineLayer oldLayer,
+    ui.EngineLayer? oldLayer,
   }) {
     final Matrix4 matrix = Matrix4.translationValues(dx, dy, 0.0);
     final TransformLayer layer = TransformLayer(matrix);
@@ -168,7 +168,7 @@ class LayerSceneBuilder implements ui.SceneBuilder {
   @override
   ui.OpacityEngineLayer pushOpacity(
     int alpha, {
-    ui.EngineLayer oldLayer,
+    ui.EngineLayer? oldLayer,
     ui.Offset offset = ui.Offset.zero,
   }) {
     final OpacityLayer layer = OpacityLayer(alpha, offset);
@@ -178,12 +178,12 @@ class LayerSceneBuilder implements ui.SceneBuilder {
 
   @override
   ui.PhysicalShapeEngineLayer pushPhysicalShape({
-    ui.Path path,
-    double elevation,
-    ui.Color color,
-    ui.Color shadowColor,
+    required ui.Path path,
+    required double elevation,
+    required ui.Color color,
+    ui.Color? shadowColor,
     ui.Clip clipBehavior = ui.Clip.none,
-    ui.EngineLayer oldLayer,
+    ui.EngineLayer? oldLayer,
   }) {
     final PhysicalShapeLayer layer =
         PhysicalShapeLayer(elevation, color, shadowColor, path, clipBehavior);
@@ -196,15 +196,15 @@ class LayerSceneBuilder implements ui.SceneBuilder {
     ui.Shader shader,
     ui.Rect maskRect,
     ui.BlendMode blendMode, {
-    ui.EngineLayer oldLayer,
+    ui.EngineLayer? oldLayer,
   }) {
     throw UnimplementedError();
   }
 
   @override
-  ui.TransformEngineLayer pushTransform(
+  ui.TransformEngineLayer? pushTransform(
     Float64List matrix4, {
-    ui.EngineLayer oldLayer,
+    ui.EngineLayer? oldLayer,
   }) {
     final Matrix4 matrix = Matrix4.fromFloat32List(toMatrix32(matrix4));
     pushLayer(TransformLayer(matrix));
@@ -236,7 +236,7 @@ class LayerSceneBuilder implements ui.SceneBuilder {
       return;
     }
 
-    currentLayer.add(layer);
+    currentLayer!.add(layer);
     currentLayer = layer;
   }
 
diff --git a/lib/web_ui/lib/src/engine/compositor/layer_tree.dart b/lib/web_ui/lib/src/engine/compositor/layer_tree.dart
index 4d9ec292982e2..00243469dcac5 100644
--- a/lib/web_ui/lib/src/engine/compositor/layer_tree.dart
+++ b/lib/web_ui/lib/src/engine/compositor/layer_tree.dart
@@ -2,19 +2,19 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A tree of [Layer]s that, together with a [Size] compose a frame.
 class LayerTree {
   /// The root of the layer tree.
-  Layer rootLayer;
+  Layer? rootLayer;
 
   /// The size (in physical pixels) of the frame to paint this layer tree into.
-  ui.Size frameSize;
+  final ui.Size frameSize = _computeFrameSize();
 
   /// The devicePixelRatio of the frame to paint this layer tree into.
-  double devicePixelRatio;
+  double? devicePixelRatio;
 
   /// Performs a preroll phase before painting the layer tree.
   ///
@@ -27,7 +27,7 @@ class LayerTree {
       ignoreRasterCache ? null : frame.rasterCache,
       frame.viewEmbedder,
     );
-    rootLayer.preroll(context, Matrix4.identity());
+    rootLayer!.preroll(context, Matrix4.identity());
   }
 
   /// Paints the layer tree into the given [frame].
@@ -37,8 +37,8 @@ class LayerTree {
   void paint(Frame frame, {bool ignoreRasterCache = false}) {
     final SkNWayCanvas internalNodesCanvas = SkNWayCanvas();
     internalNodesCanvas.addCanvas(frame.canvas);
-    final List<SkCanvas> overlayCanvases =
-        frame.viewEmbedder.getCurrentCanvases();
+    final List<SkCanvas?> overlayCanvases =
+        frame.viewEmbedder!.getCurrentCanvases();
     for (int i = 0; i < overlayCanvases.length; i++) {
       internalNodesCanvas.addCanvas(overlayCanvases[i]);
     }
@@ -48,22 +48,30 @@ class LayerTree {
       ignoreRasterCache ? null : frame.rasterCache,
       frame.viewEmbedder,
     );
-    if (rootLayer.needsPainting) {
-      rootLayer.paint(context);
+    if (rootLayer!.needsPainting) {
+      rootLayer!.paint(context);
     }
   }
 }
 
+ui.Size _computeFrameSize() {
+  final ui.Size physicalSize = ui.window.physicalSize;
+  return ui.Size(
+    physicalSize.width.truncate().toDouble(),
+    physicalSize.height.truncate().toDouble(),
+  );
+}
+
 /// A single frame to be rendered.
 class Frame {
   /// The canvas to render this frame to.
   final SkCanvas canvas;
 
   /// A cache of pre-rastered pictures.
-  final RasterCache rasterCache;
+  final RasterCache? rasterCache;
 
   /// The platform view embedder.
-  final HtmlViewEmbedder viewEmbedder;
+  final HtmlViewEmbedder? viewEmbedder;
 
   Frame(this.canvas, this.rasterCache, this.viewEmbedder);
 
@@ -82,10 +90,10 @@ class Frame {
 /// The state of the compositor, which is persisted between frames.
 class CompositorContext {
   /// A cache of pictures, which is shared between successive frames.
-  RasterCache rasterCache;
+  RasterCache? rasterCache;
 
   /// Acquire a frame using this compositor's settings.
-  Frame acquireFrame(SkCanvas canvas, HtmlViewEmbedder viewEmbedder) {
+  Frame acquireFrame(SkCanvas canvas, HtmlViewEmbedder? viewEmbedder) {
     return Frame(canvas, rasterCache, viewEmbedder);
   }
 }
diff --git a/lib/web_ui/lib/src/engine/compositor/n_way_canvas.dart b/lib/web_ui/lib/src/engine/compositor/n_way_canvas.dart
index b0ca17ae0f526..e351c2e66591a 100644
--- a/lib/web_ui/lib/src/engine/compositor/n_way_canvas.dart
+++ b/lib/web_ui/lib/src/engine/compositor/n_way_canvas.dart
@@ -2,86 +2,86 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A virtual canvas that applies operations to multiple canvases at once.
 class SkNWayCanvas {
-  final List<SkCanvas> _canvases = <SkCanvas>[];
+  final List<SkCanvas?> _canvases = <SkCanvas?>[];
 
-  void addCanvas(SkCanvas canvas) {
+  void addCanvas(SkCanvas? canvas) {
     _canvases.add(canvas);
   }
 
   /// Calls [save] on all canvases.
-  int save() {
-    int saveCount;
+  int? save() {
+    int? saveCount;
     for (int i = 0; i < _canvases.length; i++) {
-      saveCount = _canvases[i].save();
+      saveCount = _canvases[i]!.save();
     }
     return saveCount;
   }
 
   /// Calls [saveLayer] on all canvases.
-  void saveLayer(ui.Rect bounds, ui.Paint paint) {
+  void saveLayer(ui.Rect bounds, ui.Paint? paint) {
     for (int i = 0; i < _canvases.length; i++) {
-      _canvases[i].saveLayer(bounds, paint);
+      _canvases[i]!.saveLayer(bounds, paint as SkPaint);
     }
   }
 
   /// Calls [saveLayerWithFilter] on all canvases.
   void saveLayerWithFilter(ui.Rect bounds, ui.ImageFilter filter) {
     for (int i = 0; i < _canvases.length; i++) {
-      _canvases[i].saveLayerWithFilter(bounds, filter);
+      _canvases[i]!.saveLayerWithFilter(bounds, filter);
     }
   }
 
   /// Calls [restore] on all canvases.
   void restore() {
     for (int i = 0; i < _canvases.length; i++) {
-      _canvases[i].restore();
+      _canvases[i]!.restore();
     }
   }
 
   /// Calls [restoreToCount] on all canvases.
-  void restoreToCount(int count) {
+  void restoreToCount(int? count) {
     for (int i = 0; i < _canvases.length; i++) {
-      _canvases[i].restoreToCount(count);
+      _canvases[i]!.restoreToCount(count);
     }
   }
 
   /// Calls [translate] on all canvases.
   void translate(double dx, double dy) {
     for (int i = 0; i < _canvases.length; i++) {
-      _canvases[i].translate(dx, dy);
+      _canvases[i]!.translate(dx, dy);
     }
   }
 
   /// Calls [transform] on all canvases.
-  void transform(Float32List matrix) {
+  void transform(Float32List? matrix) {
     for (int i = 0; i < _canvases.length; i++) {
-      _canvases[i].transform(matrix);
+      _canvases[i]!.transform(matrix);
     }
   }
 
   /// Calls [clipPath] on all canvases.
-  void clipPath(ui.Path path, bool doAntiAlias) {
+  void clipPath(ui.Path? path, bool doAntiAlias) {
     for (int i = 0; i < _canvases.length; i++) {
-      _canvases[i].clipPath(path, doAntiAlias);
+      _canvases[i]!.clipPath(path!, doAntiAlias);
     }
   }
 
   /// Calls [clipRect] on all canvases.
   void clipRect(ui.Rect rect, ui.ClipOp clipOp, bool doAntiAlias) {
     for (int i = 0; i < _canvases.length; i++) {
-      _canvases[i].clipRect(rect, clipOp, doAntiAlias);
+      _canvases[i]!.clipRect(rect, clipOp, doAntiAlias);
     }
   }
 
   /// Calls [clipRRect] on all canvases.
   void clipRRect(ui.RRect rrect, bool doAntiAlias) {
     for (int i = 0; i < _canvases.length; i++) {
-      _canvases[i].clipRRect(rrect, doAntiAlias);
+      _canvases[i]!.clipRRect(rrect, doAntiAlias);
     }
   }
 }
diff --git a/lib/web_ui/lib/src/engine/compositor/painting.dart b/lib/web_ui/lib/src/engine/compositor/painting.dart
index 203e8662de6b6..1010018bf7f92 100644
--- a/lib/web_ui/lib/src/engine/compositor/painting.dart
+++ b/lib/web_ui/lib/src/engine/compositor/painting.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// The implementation of [ui.Paint] used by the CanvasKit backend.
@@ -13,21 +13,21 @@ class SkPaint extends SkiaObject implements ui.Paint {
   SkPaint();
 
   static const ui.Color _defaultPaintColor = ui.Color(0xFF000000);
-  static final js.JsObject _skPaintStyleStroke =
+  static final js.JsObject? _skPaintStyleStroke =
       canvasKit['PaintStyle']['Stroke'];
-  static final js.JsObject _skPaintStyleFill = canvasKit['PaintStyle']['Fill'];
+  static final js.JsObject? _skPaintStyleFill = canvasKit['PaintStyle']['Fill'];
 
   @override
   ui.BlendMode get blendMode => _blendMode;
   @override
   set blendMode(ui.BlendMode value) {
     _blendMode = value;
-    _syncBlendMode(skiaObject);
+    _syncBlendMode(skiaObject!);
   }
 
   void _syncBlendMode(js.JsObject object) {
-    final js.JsObject skBlendMode = makeSkBlendMode(_blendMode);
-    object.callMethod('setBlendMode', <js.JsObject>[skBlendMode]);
+    final js.JsObject? skBlendMode = makeSkBlendMode(_blendMode);
+    object.callMethod('setBlendMode', <js.JsObject?>[skBlendMode]);
   }
 
   ui.BlendMode _blendMode = ui.BlendMode.srcOver;
@@ -41,8 +41,8 @@ class SkPaint extends SkiaObject implements ui.Paint {
     _syncStyle(skiaObject);
   }
 
-  void _syncStyle(js.JsObject object) {
-    js.JsObject skPaintStyle;
+  void _syncStyle(js.JsObject? object) {
+    js.JsObject? skPaintStyle;
     switch (_style) {
       case ui.PaintingStyle.stroke:
         skPaintStyle = _skPaintStyleStroke;
@@ -51,7 +51,7 @@ class SkPaint extends SkiaObject implements ui.Paint {
         skPaintStyle = _skPaintStyleFill;
         break;
     }
-    object.callMethod('setStyle', <js.JsObject>[skPaintStyle]);
+    object!.callMethod('setStyle', <js.JsObject?>[skPaintStyle]);
   }
 
   ui.PaintingStyle _style = ui.PaintingStyle.fill;
@@ -61,7 +61,7 @@ class SkPaint extends SkiaObject implements ui.Paint {
   @override
   set strokeWidth(double value) {
     _strokeWidth = value;
-    _syncStrokeWidth(skiaObject);
+    _syncStrokeWidth(skiaObject!);
   }
 
   void _syncStrokeWidth(js.JsObject object) {
@@ -95,7 +95,7 @@ class SkPaint extends SkiaObject implements ui.Paint {
   @override
   set isAntiAlias(bool value) {
     _isAntiAlias = value;
-    _syncAntiAlias(skiaObject);
+    _syncAntiAlias(skiaObject!);
   }
 
   void _syncAntiAlias(js.JsObject object) {
@@ -109,16 +109,12 @@ class SkPaint extends SkiaObject implements ui.Paint {
   @override
   set color(ui.Color value) {
     _color = value;
-    _syncColor(skiaObject);
+    _syncColor(skiaObject!);
   }
 
   void _syncColor(js.JsObject object) {
-    ui.Color colorValue = _defaultPaintColor;
-    if (_color != null) {
-      colorValue = _color;
-    }
     object.callMethod('setColorInt', <int>[
-      colorValue.value,
+      _color.value,
     ]);
   }
 
@@ -135,38 +131,38 @@ class SkPaint extends SkiaObject implements ui.Paint {
   bool _invertColors = false;
 
   @override
-  ui.Shader get shader => _shader as ui.Shader;
+  ui.Shader? get shader => _shader as ui.Shader?;
   @override
-  set shader(ui.Shader value) {
-    _shader = value as EngineShader;
-    _syncShader(skiaObject);
+  set shader(ui.Shader? value) {
+    _shader = value as EngineShader?;
+    _syncShader(skiaObject!);
   }
 
   void _syncShader(js.JsObject object) {
-    js.JsObject skShader;
+    js.JsObject? skShader;
     if (_shader != null) {
-      skShader = _shader.createSkiaShader();
+      skShader = _shader!.createSkiaShader();
     }
-    object.callMethod('setShader', <js.JsObject>[skShader]);
+    object.callMethod('setShader', <js.JsObject?>[skShader]);
   }
 
-  EngineShader _shader;
+  EngineShader? _shader;
 
   @override
-  ui.MaskFilter get maskFilter => _maskFilter;
+  ui.MaskFilter? get maskFilter => _maskFilter;
   @override
-  set maskFilter(ui.MaskFilter value) {
+  set maskFilter(ui.MaskFilter? value) {
     _maskFilter = value;
     _syncMaskFilter(skiaObject);
   }
 
-  void _syncMaskFilter(js.JsObject object) {
-    js.JsObject skMaskFilter;
+  void _syncMaskFilter(js.JsObject? object) {
+    js.JsObject? skMaskFilter;
     if (_maskFilter != null) {
-      final ui.BlurStyle blurStyle = _maskFilter.webOnlyBlurStyle;
-      final double sigma = _maskFilter.webOnlySigma;
+      final ui.BlurStyle blurStyle = _maskFilter!.webOnlyBlurStyle;
+      final double sigma = _maskFilter!.webOnlySigma;
 
-      js.JsObject skBlurStyle;
+      js.JsObject? skBlurStyle;
       switch (blurStyle) {
         case ui.BlurStyle.normal:
           skBlurStyle = canvasKit['BlurStyle']['Normal'];
@@ -185,10 +181,10 @@ class SkPaint extends SkiaObject implements ui.Paint {
       skMaskFilter = canvasKit.callMethod(
           'MakeBlurMaskFilter', <dynamic>[skBlurStyle, sigma, true]);
     }
-    object.callMethod('setMaskFilter', <js.JsObject>[skMaskFilter]);
+    object!.callMethod('setMaskFilter', <js.JsObject?>[skMaskFilter]);
   }
 
-  ui.MaskFilter _maskFilter;
+  ui.MaskFilter? _maskFilter;
 
   @override
   ui.FilterQuality get filterQuality => _filterQuality;
@@ -198,8 +194,8 @@ class SkPaint extends SkiaObject implements ui.Paint {
     _syncFilterQuality(skiaObject);
   }
 
-  void _syncFilterQuality(js.JsObject object) {
-    js.JsObject skFilterQuality;
+  void _syncFilterQuality(js.JsObject? object) {
+    js.JsObject? skFilterQuality;
     switch (_filterQuality) {
       case ui.FilterQuality.none:
         skFilterQuality = canvasKit['FilterQuality']['None'];
@@ -214,29 +210,29 @@ class SkPaint extends SkiaObject implements ui.Paint {
         skFilterQuality = canvasKit['FilterQuality']['High'];
         break;
     }
-    object.callMethod('setFilterQuality', <js.JsObject>[skFilterQuality]);
+    object!.callMethod('setFilterQuality', <js.JsObject?>[skFilterQuality]);
   }
 
   ui.FilterQuality _filterQuality = ui.FilterQuality.none;
 
   @override
-  ui.ColorFilter get colorFilter => _colorFilter;
+  ui.ColorFilter? get colorFilter => _colorFilter;
   @override
-  set colorFilter(ui.ColorFilter value) {
-    _colorFilter = value;
-    _syncColorFilter(skiaObject);
+  set colorFilter(ui.ColorFilter? value) {
+    _colorFilter = value as EngineColorFilter?;
+    _syncColorFilter(skiaObject!);
   }
 
   void _syncColorFilter(js.JsObject object) {
-    js.JsObject skColorFilterJs;
+    js.JsObject? skColorFilterJs;
     if (_colorFilter != null) {
-      SkColorFilter skFilter = _colorFilter._toSkColorFilter();
+      SkColorFilter skFilter = _colorFilter!._toSkColorFilter()!;
       skColorFilterJs = skFilter.skColorFilter;
     }
-    object.callMethod('setColorFilter', <js.JsObject>[skColorFilterJs]);
+    object.callMethod('setColorFilter', <js.JsObject?>[skColorFilterJs]);
   }
 
-  EngineColorFilter _colorFilter;
+  EngineColorFilter? _colorFilter;
 
   // TODO(yjbanov): implement
   @override
@@ -249,22 +245,22 @@ class SkPaint extends SkiaObject implements ui.Paint {
   double _strokeMiterLimit = 0.0;
 
   @override
-  ui.ImageFilter get imageFilter => _imageFilter;
+  ui.ImageFilter? get imageFilter => _imageFilter;
   @override
-  set imageFilter(ui.ImageFilter value) {
-    _imageFilter = value;
-    _syncImageFilter(skiaObject);
+  set imageFilter(ui.ImageFilter? value) {
+    _imageFilter = value as SkImageFilter?;
+    _syncImageFilter(skiaObject!);
   }
 
   void _syncImageFilter(js.JsObject object) {
-    js.JsObject imageFilterJs;
+    js.JsObject? imageFilterJs;
     if (_imageFilter != null) {
-      imageFilterJs = _imageFilter.skImageFilter;
+      imageFilterJs = _imageFilter!.skImageFilter;
     }
-    object.callMethod('setImageFilter', <js.JsObject>[imageFilterJs]);
+    object.callMethod('setImageFilter', <js.JsObject?>[imageFilterJs]);
   }
 
-  SkImageFilter _imageFilter;
+  SkImageFilter? _imageFilter;
 
   @override
   js.JsObject createDefault() {
diff --git a/lib/web_ui/lib/src/engine/compositor/path.dart b/lib/web_ui/lib/src/engine/compositor/path.dart
index 1cc98e0e9c9a8..51755321c0e07 100644
--- a/lib/web_ui/lib/src/engine/compositor/path.dart
+++ b/lib/web_ui/lib/src/engine/compositor/path.dart
@@ -2,32 +2,32 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// An implementation of [ui.Path] which is backed by an `SkPath`.
 ///
 /// The `SkPath` is required for `SkCanvas` methods which take a path.
 class SkPath implements ui.Path {
-  js.JsObject _skPath;
+  js.JsObject? _skPath;
 
   /// Cached constructor function for `SkPath`, so we don't have to look it up
   /// every time we construct a new path.
-  static final js.JsFunction _skPathConstructor = canvasKit['SkPath'];
+  static final js.JsFunction? _skPathConstructor = canvasKit['SkPath'];
 
   SkPath() {
-    _skPath = js.JsObject(_skPathConstructor);
+    _skPath = js.JsObject(_skPathConstructor!);
     fillType = ui.PathFillType.nonZero;
   }
 
   SkPath.from(SkPath other) {
-    _skPath = js.JsObject(_skPathConstructor, <js.JsObject>[other._skPath]);
+    _skPath = js.JsObject(_skPathConstructor!, <js.JsObject?>[other._skPath]);
     fillType = other.fillType;
   }
 
-  SkPath._fromSkPath(js.JsObject skPath) : _skPath = skPath;
+  SkPath._fromSkPath(js.JsObject? skPath) : _skPath = skPath;
 
-  ui.PathFillType _fillType;
+  late ui.PathFillType _fillType;
 
   @override
   ui.PathFillType get fillType => _fillType;
@@ -36,7 +36,7 @@ class SkPath implements ui.Path {
   set fillType(ui.PathFillType newFillType) {
     _fillType = newFillType;
 
-    js.JsObject skFillType;
+    js.JsObject? skFillType;
     switch (newFillType) {
       case ui.PathFillType.nonZero:
         skFillType = canvasKit['FillType']['Winding'];
@@ -46,13 +46,13 @@ class SkPath implements ui.Path {
         break;
     }
 
-    _skPath.callMethod('setFillType', <js.JsObject>[skFillType]);
+    _skPath!.callMethod('setFillType', <js.JsObject?>[skFillType]);
   }
 
   @override
   void addArc(ui.Rect oval, double startAngle, double sweepAngle) {
     const double toDegrees = 180.0 / math.pi;
-    _skPath.callMethod('addArc', <dynamic>[
+    _skPath!.callMethod('addArc', <dynamic>[
       makeSkRect(oval),
       startAngle * toDegrees,
       sweepAngle * toDegrees,
@@ -61,11 +61,11 @@ class SkPath implements ui.Path {
 
   @override
   void addOval(ui.Rect oval) {
-    _skPath.callMethod('addOval', <dynamic>[makeSkRect(oval), false, 1]);
+    _skPath!.callMethod('addOval', <dynamic>[makeSkRect(oval), false, 1]);
   }
 
   @override
-  void addPath(ui.Path path, ui.Offset offset, {Float64List matrix4}) {
+  void addPath(ui.Path path, ui.Offset offset, {Float64List? matrix4}) {
     List<double> skMatrix;
     if (matrix4 == null) {
       skMatrix = makeSkMatrixFromFloat32(
@@ -75,8 +75,8 @@ class SkPath implements ui.Path {
       skMatrix[2] += offset.dx;
       skMatrix[5] += offset.dy;
     }
-    final SkPath otherPath = path;
-    _skPath.callMethod('addPath', <dynamic>[
+    final SkPath otherPath = path as SkPath;
+    _skPath!.callMethod('addPath', <dynamic>[
       otherPath._skPath,
       skMatrix[0],
       skMatrix[1],
@@ -93,10 +93,10 @@ class SkPath implements ui.Path {
 
   @override
   void addPolygon(List<ui.Offset> points, bool close) {
-    assert(points != null);
+    assert(points != null); // ignore: unnecessary_null_comparison
     // TODO(hterkelsen): https://github.com/flutter/flutter/issues/58824
-    final List<List<double>> encodedPoints = encodePointList(points);
-    _skPath.callMethod('addPoly', <dynamic>[encodedPoints, close]);
+    final List<List<double>>? encodedPoints = encodePointList(points);
+    _skPath!.callMethod('addPoly', <dynamic>[encodedPoints, close]);
   }
 
   @override
@@ -112,20 +112,20 @@ class SkPath implements ui.Path {
       rrect.blRadiusX,
       rrect.blRadiusY,
     ];
-    _skPath.callMethod('addRoundRect',
+    _skPath!.callMethod('addRoundRect',
         <dynamic>[skRect, js.JsArray<double>.from(radii), false]);
   }
 
   @override
   void addRect(ui.Rect rect) {
-    _skPath.callMethod('addRect', <js.JsObject>[makeSkRect(rect)]);
+    _skPath!.callMethod('addRect', <js.JsObject>[makeSkRect(rect)]);
   }
 
   @override
   void arcTo(
       ui.Rect rect, double startAngle, double sweepAngle, bool forceMoveTo) {
     const double toDegrees = 180.0 / math.pi;
-    _skPath.callMethod('arcTo', <dynamic>[
+    _skPath!.callMethod('arcTo', <dynamic>[
       makeSkRect(rect),
       startAngle * toDegrees,
       sweepAngle * toDegrees,
@@ -139,7 +139,7 @@ class SkPath implements ui.Path {
       double rotation = 0.0,
       bool largeArc = false,
       bool clockwise = true}) {
-    _skPath.callMethod('arcTo', <dynamic>[
+    _skPath!.callMethod('arcTo', <dynamic>[
       radius.x,
       radius.y,
       rotation,
@@ -152,7 +152,7 @@ class SkPath implements ui.Path {
 
   @override
   void close() {
-    _skPath.callMethod('close');
+    _skPath!.callMethod('close');
   }
 
   @override
@@ -162,22 +162,22 @@ class SkPath implements ui.Path {
 
   @override
   void conicTo(double x1, double y1, double x2, double y2, double w) {
-    _skPath.callMethod('conicTo', <double>[x1, y1, x2, y2, w]);
+    _skPath!.callMethod('conicTo', <double>[x1, y1, x2, y2, w]);
   }
 
   @override
   bool contains(ui.Offset point) {
-    return _skPath.callMethod('contains', <double>[point.dx, point.dy]);
+    return _skPath!.callMethod('contains', <double>[point.dx, point.dy]);
   }
 
   @override
   void cubicTo(
       double x1, double y1, double x2, double y2, double x3, double y3) {
-    _skPath.callMethod('cubicTo', <double>[x1, y1, x2, y2, x3, y3]);
+    _skPath!.callMethod('cubicTo', <double>[x1, y1, x2, y2, x3, y3]);
   }
 
   @override
-  void extendWithPath(ui.Path path, ui.Offset offset, {Float64List matrix4}) {
+  void extendWithPath(ui.Path path, ui.Offset offset, {Float64List? matrix4}) {
     List<double> skMatrix;
     if (matrix4 == null) {
       skMatrix = makeSkMatrixFromFloat32(
@@ -187,8 +187,8 @@ class SkPath implements ui.Path {
       skMatrix[2] += offset.dx;
       skMatrix[5] += offset.dy;
     }
-    final SkPath otherPath = path;
-    _skPath.callMethod('addPath', <dynamic>[
+    final SkPath otherPath = path as SkPath;
+    _skPath!.callMethod('addPath', <dynamic>[
       otherPath._skPath,
       skMatrix[0],
       skMatrix[1],
@@ -205,23 +205,23 @@ class SkPath implements ui.Path {
 
   @override
   ui.Rect getBounds() {
-    final js.JsObject bounds = _skPath.callMethod('getBounds');
+    final js.JsObject bounds = _skPath!.callMethod('getBounds');
     return fromSkRect(bounds);
   }
 
   @override
   void lineTo(double x, double y) {
-    _skPath.callMethod('lineTo', <double>[x, y]);
+    _skPath!.callMethod('lineTo', <double>[x, y]);
   }
 
   @override
   void moveTo(double x, double y) {
-    _skPath.callMethod('moveTo', <double>[x, y]);
+    _skPath!.callMethod('moveTo', <double>[x, y]);
   }
 
   @override
   void quadraticBezierTo(double x1, double y1, double x2, double y2) {
-    _skPath.callMethod('quadTo', <double>[x1, y1, x2, y2]);
+    _skPath!.callMethod('quadTo', <double>[x1, y1, x2, y2]);
   }
 
   @override
@@ -230,7 +230,7 @@ class SkPath implements ui.Path {
       double rotation = 0.0,
       bool largeArc = false,
       bool clockwise = true}) {
-    _skPath.callMethod('rArcTo', <dynamic>[
+    _skPath!.callMethod('rArcTo', <dynamic>[
       radius.x,
       radius.y,
       rotation,
@@ -243,40 +243,40 @@ class SkPath implements ui.Path {
 
   @override
   void relativeConicTo(double x1, double y1, double x2, double y2, double w) {
-    _skPath.callMethod('rConicTo', <double>[x1, y1, x2, y2, w]);
+    _skPath!.callMethod('rConicTo', <double>[x1, y1, x2, y2, w]);
   }
 
   @override
   void relativeCubicTo(
       double x1, double y1, double x2, double y2, double x3, double y3) {
-    _skPath.callMethod('rCubicTo', <double>[x1, y1, x2, y2, x3, y3]);
+    _skPath!.callMethod('rCubicTo', <double>[x1, y1, x2, y2, x3, y3]);
   }
 
   @override
   void relativeLineTo(double dx, double dy) {
-    _skPath.callMethod('rLineTo', <double>[dx, dy]);
+    _skPath!.callMethod('rLineTo', <double>[dx, dy]);
   }
 
   @override
   void relativeMoveTo(double dx, double dy) {
-    _skPath.callMethod('rMoveTo', <double>[dx, dy]);
+    _skPath!.callMethod('rMoveTo', <double>[dx, dy]);
   }
 
   @override
   void relativeQuadraticBezierTo(double x1, double y1, double x2, double y2) {
-    _skPath.callMethod('rQuadTo', <double>[x1, y1, x2, y2]);
+    _skPath!.callMethod('rQuadTo', <double>[x1, y1, x2, y2]);
   }
 
   @override
   void reset() {
-    _skPath.callMethod('reset');
+    _skPath!.callMethod('reset');
   }
 
   @override
   ui.Path shift(ui.Offset offset) {
     // Since CanvasKit does not expose `SkPath.offset`, create a copy of this
     // path and call `transform` on it.
-    final js.JsObject newPath = _skPath.callMethod('copy');
+    final js.JsObject newPath = _skPath!.callMethod('copy');
     newPath.callMethod('transform',
         <double>[1.0, 0.0, offset.dx, 0.0, 1.0, offset.dy, 0.0, 0.0, 0.0]);
     return SkPath._fromSkPath(newPath);
@@ -287,9 +287,9 @@ class SkPath implements ui.Path {
     ui.Path uiPath1,
     ui.Path uiPath2,
   ) {
-    final SkPath path1 = uiPath1;
-    final SkPath path2 = uiPath2;
-    js.JsObject pathOp;
+    final SkPath path1 = uiPath1 as SkPath;
+    final SkPath path2 = uiPath2 as SkPath;
+    js.JsObject? pathOp;
     switch (operation) {
       case ui.PathOperation.difference:
         pathOp = canvasKit['PathOp']['Difference'];
@@ -307,9 +307,9 @@ class SkPath implements ui.Path {
         pathOp = canvasKit['PathOp']['ReverseDifference'];
         break;
     }
-    final js.JsObject newPath = canvasKit.callMethod(
+    final js.JsObject? newPath = canvasKit.callMethod(
       'MakePathFromOp',
-      <js.JsObject>[
+      <js.JsObject?>[
         path1._skPath,
         path2._skPath,
         pathOp,
@@ -320,17 +320,17 @@ class SkPath implements ui.Path {
 
   @override
   ui.Path transform(Float64List matrix4) {
-    final js.JsObject newPath = _skPath.callMethod('copy');
+    final js.JsObject newPath = _skPath!.callMethod('copy');
     newPath.callMethod('transform', <js.JsArray>[makeSkMatrixFromFloat64(matrix4)]);
     return SkPath._fromSkPath(newPath);
   }
 
-  String toSvgString() {
-    return _skPath.callMethod('toSVGString');
+  String? toSvgString() {
+    return _skPath!.callMethod('toSVGString');
   }
 
   /// Return `true` if this path contains no segments.
-  bool get isEmpty {
-    return _skPath.callMethod('isEmpty');
+  bool? get isEmpty {
+    return _skPath!.callMethod('isEmpty');
   }
 }
diff --git a/lib/web_ui/lib/src/engine/compositor/path_metrics.dart b/lib/web_ui/lib/src/engine/compositor/path_metrics.dart
index 0913544bdbd23..f42ccf222eeb5 100644
--- a/lib/web_ui/lib/src/engine/compositor/path_metrics.dart
+++ b/lib/web_ui/lib/src/engine/compositor/path_metrics.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
 class SkPathMetrics extends IterableBase<ui.PathMetric>
@@ -14,16 +13,16 @@ class SkPathMetrics extends IterableBase<ui.PathMetric>
 
   /// The [SkPath.isEmpty] case is special-cased to avoid booting the WASM machinery just to find out there are no contours.
   @override
-  Iterator<ui.PathMetric> get iterator => _path.isEmpty ? const SkPathMetricIteratorEmpty._() : SkContourMeasureIter(_path, _forceClosed);
+  Iterator<ui.PathMetric> get iterator => _path.isEmpty! ? const SkPathMetricIteratorEmpty._() : SkContourMeasureIter(_path, _forceClosed);
 }
 
 class SkContourMeasureIter implements Iterator<ui.PathMetric> {
   /// Cached constructor function for `SkContourMeasureIter`, so we don't have to look it
   /// up every time we're constructing a new instance.
-  static final js.JsFunction _skContourMeasureIterConstructor = canvasKit['SkContourMeasureIter'];
+  static final js.JsFunction? _skContourMeasureIterConstructor = canvasKit['SkContourMeasureIter'];
 
   SkContourMeasureIter(SkPath path, bool forceClosed)
-    : _skObject = js.JsObject(_skContourMeasureIterConstructor, <dynamic>[
+    : _skObject = js.JsObject(_skContourMeasureIterConstructor!, <dynamic>[
         path._skPath,
         forceClosed,
         1,
@@ -38,12 +37,22 @@ class SkContourMeasureIter implements Iterator<ui.PathMetric> {
   int _contourIndexCounter = 0;
 
   @override
-  ui.PathMetric get current => _current;
-  SkContourMeasure _current;
+  ui.PathMetric get current {
+    final ui.PathMetric? currentMetric = _current;
+    if (currentMetric == null) {
+      throw RangeError(
+        'PathMetricIterator is not pointing to a PathMetric. This can happen in two situations:\n'
+        '- The iteration has not started yet. If so, call "moveNext" to start iteration.'
+        '- The iterator ran out of elements. If so, check that "moveNext" returns true prior to calling "current".'
+      );
+    }
+    return currentMetric;
+  }
+  SkContourMeasure? _current;
 
   @override
   bool moveNext() {
-    final js.JsObject skContourMeasure = _skObject.callMethod('next');
+    final js.JsObject? skContourMeasure = _skObject.callMethod('next');
     if (skContourMeasure == null) {
       _current = null;
       return false;
@@ -65,7 +74,7 @@ class SkContourMeasure implements ui.PathMetric {
 
   @override
   ui.Path extractPath(double start, double end, {bool startWithMoveTo = true}) {
-    final js.JsObject skPath = _skObject
+    final js.JsObject? skPath = _skObject
         .callMethod('getSegment', <dynamic>[start, end, startWithMoveTo]);
     return SkPath._fromSkPath(skPath);
   }
@@ -94,7 +103,9 @@ class SkPathMetricIteratorEmpty implements Iterator<ui.PathMetric> {
   const SkPathMetricIteratorEmpty._();
 
   @override
-  ui.PathMetric get current => null;
+  ui.PathMetric get current {
+    throw RangeError('PathMetric iterator is empty.');
+  }
 
   @override
   bool moveNext() {
diff --git a/lib/web_ui/lib/src/engine/compositor/picture.dart b/lib/web_ui/lib/src/engine/compositor/picture.dart
index e627d19d9fb0c..817f9b187a761 100644
--- a/lib/web_ui/lib/src/engine/compositor/picture.dart
+++ b/lib/web_ui/lib/src/engine/compositor/picture.dart
@@ -2,12 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
 class SkPicture implements ui.Picture {
-  final js.JsObject skPicture;
-  final ui.Rect cullRect;
+  final js.JsObject? skPicture;
+  final ui.Rect? cullRect;
 
   SkPicture(this.skPicture, this.cullRect);
 
@@ -21,7 +20,6 @@ class SkPicture implements ui.Picture {
 
   @override
   Future<ui.Image> toImage(int width, int height) {
-    // TODO: implement toImage
-    return null;
+    throw UnsupportedError('Picture.toImage not yet implemented for CanvasKit and HTML');
   }
 }
diff --git a/lib/web_ui/lib/src/engine/compositor/picture_recorder.dart b/lib/web_ui/lib/src/engine/compositor/picture_recorder.dart
index 81a9163d6b800..15722eee3e557 100644
--- a/lib/web_ui/lib/src/engine/compositor/picture_recorder.dart
+++ b/lib/web_ui/lib/src/engine/compositor/picture_recorder.dart
@@ -2,32 +2,32 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 class SkPictureRecorder implements ui.PictureRecorder {
-  ui.Rect _cullRect;
-  js.JsObject _recorder;
-  SkCanvas _recordingCanvas;
+  ui.Rect? _cullRect;
+  js.JsObject? _recorder;
+  SkCanvas? _recordingCanvas;
 
-  SkCanvas beginRecording(ui.Rect bounds) {
+  SkCanvas? beginRecording(ui.Rect bounds) {
     _cullRect = bounds;
     _recorder = js.JsObject(canvasKit['SkPictureRecorder']);
     final js.JsObject skRect = js.JsObject(canvasKit['LTRBRect'],
         <double>[bounds.left, bounds.top, bounds.right, bounds.bottom]);
-    final js.JsObject/*!*/ skCanvas =
-        _recorder.callMethod('beginRecording', <js.JsObject>[skRect]);
+    final js.JsObject skCanvas =
+        _recorder!.callMethod('beginRecording', <js.JsObject>[skRect]);
     _recordingCanvas = SkCanvas(skCanvas);
     return _recordingCanvas;
   }
 
-  SkCanvas get recordingCanvas => _recordingCanvas;
+  SkCanvas? get recordingCanvas => _recordingCanvas;
 
   @override
   ui.Picture endRecording() {
-    final js.JsObject skPicture =
-        _recorder.callMethod('finishRecordingAsPicture');
-    _recorder.callMethod('delete');
+    final js.JsObject? skPicture =
+        _recorder!.callMethod('finishRecordingAsPicture');
+    _recorder!.callMethod('delete');
     _recorder = null;
     return SkPicture(skPicture, _cullRect);
   }
diff --git a/lib/web_ui/lib/src/engine/compositor/platform_message.dart b/lib/web_ui/lib/src/engine/compositor/platform_message.dart
index 0b925e368b554..f7f1228241e8d 100644
--- a/lib/web_ui/lib/src/engine/compositor/platform_message.dart
+++ b/lib/web_ui/lib/src/engine/compositor/platform_message.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 class PlatformMessage {
diff --git a/lib/web_ui/lib/src/engine/compositor/raster_cache.dart b/lib/web_ui/lib/src/engine/compositor/raster_cache.dart
index 764129e10a99e..14f8fa954a10b 100644
--- a/lib/web_ui/lib/src/engine/compositor/raster_cache.dart
+++ b/lib/web_ui/lib/src/engine/compositor/raster_cache.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A cache of [Picture]s that have already been rasterized.
diff --git a/lib/web_ui/lib/src/engine/compositor/rasterizer.dart b/lib/web_ui/lib/src/engine/compositor/rasterizer.dart
index af9df107f1deb..9cebae49be9ca 100644
--- a/lib/web_ui/lib/src/engine/compositor/rasterizer.dart
+++ b/lib/web_ui/lib/src/engine/compositor/rasterizer.dart
@@ -2,38 +2,25 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A class that can rasterize [LayerTree]s into a given [Surface].
 class Rasterizer {
   final Surface surface;
   final CompositorContext context = CompositorContext();
-  final HtmlViewEmbedder viewEmbedder = HtmlViewEmbedder();
   final List<ui.VoidCallback> _postFrameCallbacks = <ui.VoidCallback>[];
 
-  Rasterizer(this.surface) {
-    surface.viewEmbedder = viewEmbedder;
-  }
+  Rasterizer(this.surface);
 
   /// Creates a new frame from this rasterizer's surface, draws the given
   /// [LayerTree] into it, and then submits the frame.
   void draw(LayerTree layerTree) {
     try {
-      if (layerTree == null) {
-        return;
-      }
-
-      final ui.Size physicalSize = ui.window.physicalSize;
-      final ui.Size frameSize = ui.Size(
-        physicalSize.width.truncate().toDouble(),
-        physicalSize.height.truncate().toDouble(),
-      );
-
-      if (frameSize.isEmpty) {
+      if (layerTree.frameSize.isEmpty) {
+        // Available drawing area is empty. Skip drawing.
         return;
       }
-      layerTree.frameSize = frameSize;
 
       final SurfaceFrame frame = surface.acquireFrame(layerTree.frameSize);
       surface.viewEmbedder.frameSize = layerTree.frameSize;
diff --git a/lib/web_ui/lib/src/engine/compositor/surface.dart b/lib/web_ui/lib/src/engine/compositor/surface.dart
index 38ad4606d0188..9d54f37adfae2 100644
--- a/lib/web_ui/lib/src/engine/compositor/surface.dart
+++ b/lib/web_ui/lib/src/engine/compositor/surface.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
 typedef SubmitCallback = bool Function(SurfaceFrame, SkCanvas);
@@ -15,7 +14,8 @@ class SurfaceFrame {
 
   SurfaceFrame(this.skiaSurface, this.submitCallback)
       : _submitted = false,
-        assert(submitCallback != null);
+        assert(skiaSurface != null), // ignore: unnecessary_null_comparison
+        assert(submitCallback != null); // ignore: unnecessary_null_comparison
 
   /// Submit this frame to be drawn.
   bool submit() {
@@ -25,7 +25,7 @@ class SurfaceFrame {
     return submitCallback(this, skiaCanvas);
   }
 
-  SkCanvas get skiaCanvas => skiaSurface?.getCanvas();
+  SkCanvas get skiaCanvas => skiaSurface.getCanvas();
 }
 
 /// A surface which can be drawn into by the compositor.
@@ -34,76 +34,67 @@ class SurfaceFrame {
 /// successive frames if they are the same size. Otherwise, a new [SkSurface] is
 /// created.
 class Surface {
-  SkSurface _surface;
-  html.Element htmlElement;
+  Surface(this.viewEmbedder);
+
+  SkSurface? _surface;
+  html.Element? htmlElement;
 
   bool _addedToScene = false;
 
   /// The default view embedder. Coordinates embedding platform views and
   /// overlaying subsequent draw operations on top.
-  HtmlViewEmbedder viewEmbedder;
+  final HtmlViewEmbedder viewEmbedder;
 
   /// Acquire a frame of the given [size] containing a drawable canvas.
   ///
   /// The given [size] is in physical pixels.
   SurfaceFrame acquireFrame(ui.Size size) {
     final SkSurface surface = acquireRenderSurface(size);
-    canvasKit.callMethod('setCurrentContext', <int>[surface.context]);
-
-    if (surface == null) {
-      return null;
-    }
 
+    canvasKit.callMethod('setCurrentContext', <int?>[surface.context]);
     SubmitCallback submitCallback =
         (SurfaceFrame surfaceFrame, SkCanvas canvas) {
-      return _presentSurface(canvas);
+      return _presentSurface();
     };
 
     return SurfaceFrame(surface, submitCallback);
   }
 
   SkSurface acquireRenderSurface(ui.Size size) {
-    if (!_createOrUpdateSurfaces(size)) {
-      return null;
-    }
-    return _surface;
+    _createOrUpdateSurfaces(size);
+    return _surface!;
   }
 
   void addToScene() {
     if (!_addedToScene) {
-      skiaSceneHost.children.insert(0, htmlElement);
+      skiaSceneHost!.children.insert(0, htmlElement!);
     }
     _addedToScene = true;
   }
 
-  bool _createOrUpdateSurfaces(ui.Size size) {
-    if (_surface != null &&
-        size ==
-            ui.Size(
-              _surface.width().toDouble(),
-              _surface.height().toDouble(),
-            )) {
-      return true;
+  void _createOrUpdateSurfaces(ui.Size size) {
+    if (size.isEmpty) {
+      throw CanvasKitError('Cannot create surfaces of empty size.');
+    }
+
+    final SkSurface? currentSurface = _surface;
+    if (currentSurface != null) {
+      final bool isSameSize =
+        size.width == currentSurface.width() &&
+        size.height == currentSurface.height();
+      if (isSameSize) {
+        // The existing surface is still reusable.
+        return;
+      }
     }
 
-    _surface?.dispose();
+    currentSurface?.dispose();
     _surface = null;
     htmlElement?.remove();
     htmlElement = null;
     _addedToScene = false;
 
-    if (size.isEmpty) {
-      html.window.console.error('Cannot create surfaces of empty size.');
-      return false;
-    }
     _surface = _wrapHtmlCanvas(size);
-
-    if (_surface == null) {
-      html.window.console.error('Could not create a surface.');
-      return false;
-    }
-
-    return true;
   }
 
   SkSurface _wrapHtmlCanvas(ui.Size size) {
@@ -120,9 +111,14 @@ class Surface {
       // anti-aliased by setting their `Paint` object's `antialias` property.
       js.JsObject.jsify({'antialias': 0}),
     ]);
-    final js.JsObject grContext =
+    final js.JsObject? grContext =
         canvasKit.callMethod('MakeGrContext', <dynamic>[glContext]);
-    final js.JsObject skSurface =
+
+    if (grContext == null) {
+      throw CanvasKitError('Could not create a graphics context.');
+    }
+
+    final js.JsObject? skSurface =
         canvasKit.callMethod('MakeOnScreenGLSurface', <dynamic>[
       grContext,
       size.width,
@@ -130,22 +126,17 @@ class Surface {
       canvasKit['SkColorSpace']['SRGB'],
     ]);
 
-    htmlElement = htmlCanvas;
-
     if (skSurface == null) {
-      return null;
-    } else {
-      return SkSurface(skSurface, glContext);
+      throw CanvasKitError('Could not create a surface.');
     }
-  }
 
-  bool _presentSurface(SkCanvas canvas) {
-    if (canvas == null) {
-      return false;
-    }
+    htmlElement = htmlCanvas;
+    return SkSurface(skSurface, glContext);
+  }
 
-    canvasKit.callMethod('setCurrentContext', <dynamic>[_surface.context]);
-    _surface.getCanvas().flush();
+  bool _presentSurface() {
+    canvasKit.callMethod('setCurrentContext', <dynamic>[_surface!.context]);
+    _surface!.getCanvas().flush();
     return true;
   }
 }
@@ -158,7 +149,7 @@ class SkSurface {
   SkSurface(this._surface, this._glContext);
 
   SkCanvas getCanvas() {
-    final js.JsObject/*!*/ skCanvas = _surface.callMethod('getCanvas');
+    final js.JsObject skCanvas = _surface.callMethod('getCanvas');
     return SkCanvas(skCanvas);
   }
 
diff --git a/lib/web_ui/lib/src/engine/compositor/text.dart b/lib/web_ui/lib/src/engine/compositor/text.dart
index 1e53d0e0cbbf2..8b8be4914c500 100644
--- a/lib/web_ui/lib/src/engine/compositor/text.dart
+++ b/lib/web_ui/lib/src/engine/compositor/text.dart
@@ -1,23 +1,23 @@
 // Copyright 2013 The Flutter Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
-// @dart = 2.6
+
 part of engine;
 
 class SkParagraphStyle implements ui.ParagraphStyle {
   SkParagraphStyle({
-    ui.TextAlign textAlign,
-    ui.TextDirection textDirection,
-    int maxLines,
-    String fontFamily,
-    double fontSize,
-    double height,
-    ui.TextHeightBehavior textHeightBehavior,
-    ui.FontWeight fontWeight,
-    ui.FontStyle fontStyle,
-    ui.StrutStyle strutStyle,
-    String ellipsis,
-    ui.Locale locale,
+    ui.TextAlign? textAlign,
+    ui.TextDirection? textDirection,
+    int? maxLines,
+    String? fontFamily,
+    double? fontSize,
+    double? height,
+    ui.TextHeightBehavior? textHeightBehavior,
+    ui.FontWeight? fontWeight,
+    ui.FontStyle? fontStyle,
+    ui.StrutStyle? strutStyle,
+    String? ellipsis,
+    ui.Locale? locale,
   }) {
     skParagraphStyle = toSkParagraphStyle(
       textAlign,
@@ -36,15 +36,15 @@ class SkParagraphStyle implements ui.ParagraphStyle {
     _fontFamily = fontFamily;
   }
 
-  js.JsObject skParagraphStyle;
-  ui.TextDirection _textDirection;
-  String _fontFamily;
+  js.JsObject? skParagraphStyle;
+  ui.TextDirection? _textDirection;
+  String? _fontFamily;
 
   static Map<String, dynamic> toSkTextStyle(
-    String fontFamily,
-    double fontSize,
-    ui.FontWeight fontWeight,
-    ui.FontStyle fontStyle,
+    String? fontFamily,
+    double? fontSize,
+    ui.FontWeight? fontWeight,
+    ui.FontStyle? fontStyle,
   ) {
     final Map<String, dynamic> skTextStyle = <String, dynamic>{};
     if (fontWeight != null || fontStyle != null) {
@@ -67,17 +67,17 @@ class SkParagraphStyle implements ui.ParagraphStyle {
     return skTextStyle;
   }
 
-  static js.JsObject toSkParagraphStyle(
-    ui.TextAlign textAlign,
-    ui.TextDirection textDirection,
-    int maxLines,
-    String fontFamily,
-    double fontSize,
-    double height,
-    ui.TextHeightBehavior textHeightBehavior,
-    ui.FontWeight fontWeight,
-    ui.FontStyle fontStyle,
-    String ellipsis,
+  static js.JsObject? toSkParagraphStyle(
+    ui.TextAlign? textAlign,
+    ui.TextDirection? textDirection,
+    int? maxLines,
+    String? fontFamily,
+    double? fontSize,
+    double? height,
+    ui.TextHeightBehavior? textHeightBehavior,
+    ui.FontWeight? fontWeight,
+    ui.FontStyle? fontStyle,
+    String? ellipsis,
   ) {
     final Map<String, dynamic> skParagraphStyle = <String, dynamic>{};
 
@@ -140,28 +140,28 @@ class SkParagraphStyle implements ui.ParagraphStyle {
 }
 
 class SkTextStyle implements ui.TextStyle {
-  js.JsObject skTextStyle;
+  js.JsObject? skTextStyle;
 
   SkTextStyle({
-    ui.Color color,
-    ui.TextDecoration decoration,
-    ui.Color decorationColor,
-    ui.TextDecorationStyle decorationStyle,
-    double decorationThickness,
-    ui.FontWeight fontWeight,
-    ui.FontStyle fontStyle,
-    ui.TextBaseline textBaseline,
-    String fontFamily,
-    List<String> fontFamilyFallback,
-    double fontSize,
-    double letterSpacing,
-    double wordSpacing,
-    double height,
-    ui.Locale locale,
-    SkPaint background,
-    SkPaint foreground,
-    List<ui.Shadow> shadows,
-    List<ui.FontFeature> fontFeatures,
+    ui.Color? color,
+    ui.TextDecoration? decoration,
+    ui.Color? decorationColor,
+    ui.TextDecorationStyle? decorationStyle,
+    double? decorationThickness,
+    ui.FontWeight? fontWeight,
+    ui.FontStyle? fontStyle,
+    ui.TextBaseline? textBaseline,
+    String? fontFamily,
+    List<String>? fontFamilyFallback,
+    double? fontSize,
+    double? letterSpacing,
+    double? wordSpacing,
+    double? height,
+    ui.Locale? locale,
+    SkPaint? background,
+    SkPaint? foreground,
+    List<ui.Shadow>? shadows,
+    List<ui.FontFeature>? fontFeatures,
   }) {
     final js.JsObject style = js.JsObject(js.context['Object']);
 
@@ -203,7 +203,7 @@ class SkTextStyle implements ui.TextStyle {
     if (skiaFontCollection.fontFamilyOverrides.containsKey(fontFamily)) {
       fontFamily = skiaFontCollection.fontFamilyOverrides[fontFamily];
     }
-    List<String> fontFamilies = <String>[fontFamily];
+    List<String?> fontFamilies = <String?>[fontFamily];
     if (fontFamilyFallback != null &&
         !fontFamilyFallback.every((font) => fontFamily == font)) {
       fontFamilies.addAll(fontFamilyFallback);
@@ -234,9 +234,9 @@ class SkTextStyle implements ui.TextStyle {
   }
 }
 
-Map<String, js.JsObject> toSkFontStyle(
-    ui.FontWeight fontWeight, ui.FontStyle fontStyle) {
-  Map<String, js.JsObject> style = <String, js.JsObject>{};
+Map<String, js.JsObject?> toSkFontStyle(
+    ui.FontWeight? fontWeight, ui.FontStyle? fontStyle) {
+  Map<String, js.JsObject?> style = <String, js.JsObject?>{};
   if (fontWeight != null) {
     switch (fontWeight) {
       case ui.FontWeight.w100:
@@ -285,37 +285,37 @@ Map<String, js.JsObject> toSkFontStyle(
 class SkParagraph implements ui.Paragraph {
   SkParagraph(this.skParagraph, this._textDirection, this._fontFamily);
 
-  final js.JsObject skParagraph;
-  final ui.TextDirection _textDirection;
-  final String _fontFamily;
+  final js.JsObject? skParagraph;
+  final ui.TextDirection? _textDirection;
+  final String? _fontFamily;
 
   @override
   double get alphabeticBaseline =>
-      skParagraph.callMethod('getAlphabeticBaseline');
+      skParagraph!.callMethod('getAlphabeticBaseline');
 
   @override
-  bool get didExceedMaxLines => skParagraph.callMethod('didExceedMaxLines');
+  bool get didExceedMaxLines => skParagraph!.callMethod('didExceedMaxLines');
 
   @override
-  double get height => skParagraph.callMethod('getHeight');
+  double get height => skParagraph!.callMethod('getHeight');
 
   @override
   double get ideographicBaseline =>
-      skParagraph.callMethod('getIdeographicBaseline');
+      skParagraph!.callMethod('getIdeographicBaseline');
 
   @override
-  double get longestLine => skParagraph.callMethod('getLongestLine');
+  double get longestLine => skParagraph!.callMethod('getLongestLine');
 
   @override
   double get maxIntrinsicWidth =>
-      skParagraph.callMethod('getMaxIntrinsicWidth');
+      skParagraph!.callMethod('getMaxIntrinsicWidth');
 
   @override
   double get minIntrinsicWidth =>
-      skParagraph.callMethod('getMinIntrinsicWidth');
+      skParagraph!.callMethod('getMinIntrinsicWidth');
 
   @override
-  double get width => skParagraph.callMethod('getMaxWidth');
+  double get width => skParagraph!.callMethod('getMaxWidth');
 
   // TODO(hterkelsen): Implement placeholders once it's in CanvasKit
   @override
@@ -334,7 +334,7 @@ class SkParagraph implements ui.Paragraph {
       return const <ui.TextBox>[];
     }
 
-    js.JsObject heightStyle;
+    js.JsObject? heightStyle;
     switch (boxHeightStyle) {
       case ui.BoxHeightStyle.tight:
         heightStyle = canvasKit['RectHeightStyle']['Tight'];
@@ -350,7 +350,7 @@ class SkParagraph implements ui.Paragraph {
         break;
     }
 
-    js.JsObject widthStyle;
+    js.JsObject? widthStyle;
     switch (boxWidthStyle) {
       case ui.BoxWidthStyle.tight:
         widthStyle = canvasKit['RectWidthStyle']['Tight'];
@@ -361,24 +361,24 @@ class SkParagraph implements ui.Paragraph {
     }
 
     List<js.JsObject> skRects =
-        skParagraph.callMethod('getRectsForRange', <dynamic>[
+        skParagraph!.callMethod('getRectsForRange', <dynamic>[
       start,
       end,
       heightStyle,
       widthStyle,
     ]);
 
-    List<ui.TextBox> result = List<ui.TextBox>(skRects.length);
+    List<ui.TextBox> result = <ui.TextBox>[];
 
     for (int i = 0; i < skRects.length; i++) {
       final js.JsObject rect = skRects[i];
-      result[i] = ui.TextBox.fromLTRBD(
+      result.add(ui.TextBox.fromLTRBD(
         rect['fLeft'],
         rect['fTop'],
         rect['fRight'],
         rect['fBottom'],
-        _textDirection,
-      );
+        _textDirection!,
+      ));
     }
 
     return result;
@@ -387,7 +387,7 @@ class SkParagraph implements ui.Paragraph {
   @override
   ui.TextPosition getPositionForOffset(ui.Offset offset) {
     js.JsObject positionWithAffinity =
-        skParagraph.callMethod('getGlyphPositionAtCoordinate', <double>[
+        skParagraph!.callMethod('getGlyphPositionAtCoordinate', <double>[
       offset.dx,
       offset.dy,
     ]);
@@ -397,13 +397,13 @@ class SkParagraph implements ui.Paragraph {
   @override
   ui.TextRange getWordBoundary(ui.TextPosition position) {
     js.JsObject skRange =
-        skParagraph.callMethod('getWordBoundary', <int>[position.offset]);
+        skParagraph!.callMethod('getWordBoundary', <int>[position.offset]);
     return ui.TextRange(start: skRange['start'], end: skRange['end']);
   }
 
   @override
   void layout(ui.ParagraphConstraints constraints) {
-    assert(constraints.width != null);
+    assert(constraints.width != null); // ignore: unnecessary_null_comparison
 
     // Infinite width breaks layout, just use a very large number instead.
     // TODO(het): Remove this once https://bugs.chromium.org/p/skia/issues/detail?id=9874
@@ -418,7 +418,7 @@ class SkParagraph implements ui.Paragraph {
     // TODO(het): CanvasKit throws an exception when laid out with
     // a font that wasn't registered.
     try {
-      skParagraph.callMethod('layout', <double>[width]);
+      skParagraph!.callMethod('layout', <double>[width]);
     } catch (e) {
       html.window.console.warn('CanvasKit threw an exception while laying '
           'out the paragraph. The font was "$_fontFamily". Exception:\n$e');
@@ -440,17 +440,17 @@ class SkParagraph implements ui.Paragraph {
 }
 
 class SkParagraphBuilder implements ui.ParagraphBuilder {
-  js.JsObject _paragraphBuilder;
-  ui.TextDirection _textDirection;
-  String _fontFamily;
+  js.JsObject? _paragraphBuilder;
+  ui.TextDirection? _textDirection;
+  String? _fontFamily;
 
   SkParagraphBuilder(ui.ParagraphStyle style) {
-    SkParagraphStyle skStyle = style;
+    SkParagraphStyle skStyle = style as SkParagraphStyle;
     _textDirection = skStyle._textDirection;
     _fontFamily = skStyle._fontFamily;
     _paragraphBuilder = canvasKit['ParagraphBuilder'].callMethod(
       'Make',
-      <js.JsObject>[
+      <js.JsObject?>[
         skStyle.skParagraphStyle,
         skiaFontCollection.skFontMgr,
       ],
@@ -464,22 +464,22 @@ class SkParagraphBuilder implements ui.ParagraphBuilder {
     double height,
     ui.PlaceholderAlignment alignment, {
     double scale = 1.0,
-    double baselineOffset,
-    ui.TextBaseline baseline,
+    double? baselineOffset,
+    ui.TextBaseline? baseline,
   }) {
     throw UnimplementedError('addPlaceholder');
   }
 
   @override
   void addText(String text) {
-    _paragraphBuilder.callMethod('addText', <String>[text]);
+    _paragraphBuilder!.callMethod('addText', <String>[text]);
   }
 
   @override
   ui.Paragraph build() {
     final SkParagraph paragraph = SkParagraph(
-        _paragraphBuilder.callMethod('build'), _textDirection, _fontFamily);
-    _paragraphBuilder.callMethod('delete');
+        _paragraphBuilder!.callMethod('build'), _textDirection, _fontFamily);
+    _paragraphBuilder!.callMethod('delete');
     _paragraphBuilder = null;
     return paragraph;
   }
@@ -493,13 +493,13 @@ class SkParagraphBuilder implements ui.ParagraphBuilder {
 
   @override
   void pop() {
-    _paragraphBuilder.callMethod('pop');
+    _paragraphBuilder!.callMethod('pop');
   }
 
   @override
   void pushStyle(ui.TextStyle style) {
-    final SkTextStyle skStyle = style;
-    _paragraphBuilder
-        .callMethod('pushStyle', <js.JsObject>[skStyle.skTextStyle]);
+    final SkTextStyle skStyle = style as SkTextStyle;
+    _paragraphBuilder!
+        .callMethod('pushStyle', <js.JsObject?>[skStyle.skTextStyle]);
   }
 }
diff --git a/lib/web_ui/lib/src/engine/compositor/util.dart b/lib/web_ui/lib/src/engine/compositor/util.dart
index 008291cc2a849..0f140de94d150 100644
--- a/lib/web_ui/lib/src/engine/compositor/util.dart
+++ b/lib/web_ui/lib/src/engine/compositor/util.dart
@@ -2,9 +2,19 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
+/// An error related to the CanvasKit rendering backend.
+class CanvasKitError extends Error {
+  CanvasKitError(this.message);
+
+  /// Describes this error.
+  final String message;
+
+  @override
+  String toString() => 'CanvasKitError: $message';
+}
+
 /// An object backed by a [js.JsObject] mapped onto a Skia C++ object in the
 /// WebAssembly heap.
 ///
@@ -36,7 +46,7 @@ abstract class SkiaObject {
   }
 
   /// The JavaScript object that's mapped onto a Skia C++ object in the WebAssembly heap.
-  js.JsObject get skiaObject {
+  js.JsObject? get skiaObject {
     if (_skiaObject == null) {
       _skiaObject = resurrect();
       SkiaObjects.manage(this);
@@ -45,7 +55,7 @@ abstract class SkiaObject {
   }
 
   /// Do not use this field outside this class. Use [skiaObject] instead.
-  js.JsObject _skiaObject;
+  js.JsObject? _skiaObject;
 
   /// Instantiates a new Skia-backed JavaScript object containing default
   /// values.
@@ -66,7 +76,7 @@ class SkiaObjects {
   //                beyond a single frame.
   @visibleForTesting
   static final List<SkiaObject> managedObjects = () {
-    window.rasterizer.addPostFrameCallback(postFrameCleanUp);
+    window.rasterizer!.addPostFrameCallback(postFrameCleanUp);
     return <SkiaObject>[];
   }();
 
@@ -88,7 +98,7 @@ class SkiaObjects {
 
     for (int i = 0; i < managedObjects.length; i++) {
       final SkiaObject object = managedObjects[i];
-      object._skiaObject.callMethod('delete');
+      object._skiaObject!.callMethod('delete');
       object._skiaObject = null;
     }
 
@@ -117,9 +127,9 @@ js.JsObject _mallocColorArray() {
       .callMethod('Malloc', <dynamic>[js.context['Float32Array'], 4]);
 }
 
-js.JsObject sharedSkColor1;
-js.JsObject sharedSkColor2;
-js.JsObject sharedSkColor3;
+js.JsObject? sharedSkColor1;
+js.JsObject? sharedSkColor2;
+js.JsObject? sharedSkColor3;
 
 void _setSharedColor(js.JsObject sharedColor, ui.Color color) {
   Float32List array = sharedColor.callMethod('toTypedArray');
@@ -133,21 +143,21 @@ void setSharedSkColor1(ui.Color color) {
   if (sharedSkColor1 == null) {
     sharedSkColor1 = _mallocColorArray();
   }
-  _setSharedColor(sharedSkColor1, color);
+  _setSharedColor(sharedSkColor1!, color);
 }
 
 void setSharedSkColor2(ui.Color color) {
   if (sharedSkColor2 == null) {
     sharedSkColor2 = _mallocColorArray();
   }
-  _setSharedColor(sharedSkColor2, color);
+  _setSharedColor(sharedSkColor2!, color);
 }
 
 void setSharedSkColor3(ui.Color color) {
   if (sharedSkColor3 == null) {
     sharedSkColor3 = _mallocColorArray();
   }
-  _setSharedColor(sharedSkColor3, color);
+  _setSharedColor(sharedSkColor3!, color);
 }
 
 /// Creates a new color array.
@@ -214,7 +224,7 @@ js.JsArray<double> makeSkPoint(ui.Offset point) {
 
 // TODO(hterkelsen): https://github.com/flutter/flutter/issues/58824
 /// Creates a point list using a 2D JS array.
-js.JsArray<js.JsArray<double>> encodePointList(List<ui.Offset> points) {
+js.JsArray<js.JsArray<double>>? encodePointList(List<ui.Offset>? points) {
   if (points == null) {
     return null;
   }
@@ -236,7 +246,7 @@ js.JsArray<js.JsArray<double>> encodePointList(List<ui.Offset> points) {
 
 // TODO(hterkelsen): https://github.com/flutter/flutter/issues/58824
 /// Creates a point list using a 2D JS array.
-List<List<double>> encodeRawPointList(Float32List points) {
+List<List<double>>? encodeRawPointList(Float32List? points) {
   if (points == null) {
     return null;
   }
@@ -257,7 +267,7 @@ List<List<double>> encodeRawPointList(Float32List points) {
   return result;
 }
 
-js.JsObject makeSkPointMode(ui.PointMode pointMode) {
+js.JsObject? makeSkPointMode(ui.PointMode pointMode) {
   switch (pointMode) {
     case ui.PointMode.points:
       return canvasKit['PointMode']['Points'];
@@ -270,7 +280,7 @@ js.JsObject makeSkPointMode(ui.PointMode pointMode) {
   }
 }
 
-js.JsObject makeSkBlendMode(ui.BlendMode blendMode) {
+js.JsObject? makeSkBlendMode(ui.BlendMode? blendMode) {
   switch (blendMode) {
     case ui.BlendMode.clear:
       return canvasKit['BlendMode']['Clear'];
@@ -344,12 +354,12 @@ const List<int> _skMatrixIndexToMatrix4Index = <int>[
 
 /// Converts a 4x4 Flutter matrix (represented as a [Float32List]) to an
 /// SkMatrix, which is a 3x3 transform matrix.
-js.JsArray<double> makeSkMatrixFromFloat32(Float32List matrix4) {
+js.JsArray<double> makeSkMatrixFromFloat32(Float32List? matrix4) {
   final js.JsArray<double> skMatrix = js.JsArray<double>();
   skMatrix.length = 9;
   for (int i = 0; i < 9; ++i) {
     final int matrix4Index = _skMatrixIndexToMatrix4Index[i];
-    if (matrix4Index < matrix4.length)
+    if (matrix4Index < matrix4!.length)
       skMatrix[i] = matrix4[matrix4Index];
     else
       skMatrix[i] = 0.0;
@@ -382,7 +392,7 @@ final js.JsArray<double> _kDefaultColorStops = () {
 /// Converts a list of color stops into a Skia-compatible JS array or color stops.
 ///
 /// In Flutter `null` means two color stops `[0, 1]` that in Skia must be specified explicitly.
-js.JsArray<double> makeSkiaColorStops(List<double> colorStops) {
+js.JsArray<double> makeSkiaColorStops(List<double>? colorStops) {
   if (colorStops == null) {
     return _kDefaultColorStops;
   }
diff --git a/lib/web_ui/lib/src/engine/compositor/vertices.dart b/lib/web_ui/lib/src/engine/compositor/vertices.dart
index f93308ee841e9..0c05dca49b28c 100644
--- a/lib/web_ui/lib/src/engine/compositor/vertices.dart
+++ b/lib/web_ui/lib/src/engine/compositor/vertices.dart
@@ -2,29 +2,29 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 js.JsArray<Float32List> _encodeRawColorList(Int32List rawColors) {
   final int colorCount = rawColors.length;
-  final List<ui.Color> colors = List<ui.Color>(colorCount);
+  final List<ui.Color> colors = <ui.Color>[];
   for (int i = 0; i < colorCount; ++i) {
-    colors[i] = ui.Color(rawColors[i]);
+    colors.add(ui.Color(rawColors[i]));
   }
   return makeColorList(colors);
 }
 
 class SkVertices implements ui.Vertices {
-  js.JsObject skVertices;
+  js.JsObject? skVertices;
 
   SkVertices(
     ui.VertexMode mode,
     List<ui.Offset> positions, {
-    List<ui.Offset> textureCoordinates,
-    List<ui.Color> colors,
-    List<int> indices,
-  })  : assert(mode != null),
-        assert(positions != null) {
+    List<ui.Offset>? textureCoordinates,
+    List<ui.Color>? colors,
+    List<int>? indices,
+  })  : assert(mode != null), // ignore: unnecessary_null_comparison
+        assert(positions != null) { // ignore: unnecessary_null_comparison
     if (textureCoordinates != null &&
         textureCoordinates.length != positions.length)
       throw ArgumentError(
@@ -36,10 +36,10 @@ class SkVertices implements ui.Vertices {
       throw ArgumentError(
           '"indices" values must be valid indices in the positions list.');
 
-    final js.JsArray<js.JsArray<double>> encodedPositions = encodePointList(positions);
-    final js.JsArray<js.JsArray<double>> encodedTextures =
+    final js.JsArray<js.JsArray<double>>? encodedPositions = encodePointList(positions);
+    final js.JsArray<js.JsArray<double>>? encodedTextures =
         encodePointList(textureCoordinates);
-    final js.JsArray<Float32List> encodedColors =
+    final js.JsArray<Float32List>? encodedColors =
         colors != null ? makeColorList(colors) : null;
     if (!_init(mode, encodedPositions, encodedTextures, encodedColors, indices))
       throw ArgumentError('Invalid configuration for vertices.');
@@ -48,11 +48,11 @@ class SkVertices implements ui.Vertices {
   SkVertices.raw(
     ui.VertexMode mode,
     Float32List positions, {
-    Float32List textureCoordinates,
-    Int32List colors,
-    Uint16List indices,
-  })  : assert(mode != null),
-        assert(positions != null) {
+    Float32List? textureCoordinates,
+    Int32List? colors,
+    Uint16List? indices,
+  })  : assert(mode != null), // ignore: unnecessary_null_comparison
+        assert(positions != null) { // ignore: unnecessary_null_comparison
     if (textureCoordinates != null &&
         textureCoordinates.length != positions.length)
       throw ArgumentError(
@@ -66,9 +66,9 @@ class SkVertices implements ui.Vertices {
 
     if (!_init(
       mode,
-      encodeRawPointList(positions),
-      encodeRawPointList(textureCoordinates),
-      _encodeRawColorList(colors),
+      encodeRawPointList(positions) as js.JsArray<js.JsArray<double>>?,
+      encodeRawPointList(textureCoordinates) as js.JsArray<js.JsArray<double>>?,
+      colors != null ? _encodeRawColorList(colors) : null,
       indices,
     )) {
       throw ArgumentError('Invalid configuration for vertices.');
@@ -77,11 +77,11 @@ class SkVertices implements ui.Vertices {
 
   bool _init(
       ui.VertexMode mode,
-      js.JsArray<js.JsArray<double>> positions,
-      js.JsArray<js.JsArray<double>> textureCoordinates,
-      js.JsArray<Float32List> colors,
-      List<int> indices) {
-    js.JsObject skVertexMode;
+      js.JsArray<js.JsArray<double>>? positions,
+      js.JsArray<js.JsArray<double>>? textureCoordinates,
+      js.JsArray<Float32List>? colors,
+      List<int>? indices) {
+    js.JsObject? skVertexMode;
     switch (mode) {
       case ui.VertexMode.triangles:
         skVertexMode = canvasKit['VertexMode']['Triangles'];
@@ -94,7 +94,7 @@ class SkVertices implements ui.Vertices {
         break;
     }
 
-    final js.JsObject vertices =
+    final js.JsObject? vertices =
         canvasKit.callMethod('MakeSkVertices', <dynamic>[
       skVertexMode,
       positions,
diff --git a/lib/web_ui/lib/src/engine/compositor/viewport_metrics.dart b/lib/web_ui/lib/src/engine/compositor/viewport_metrics.dart
index 13cd698db89a9..a36a9906566ff 100644
--- a/lib/web_ui/lib/src/engine/compositor/viewport_metrics.dart
+++ b/lib/web_ui/lib/src/engine/compositor/viewport_metrics.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 class ViewportMetrics {
diff --git a/lib/web_ui/lib/src/engine/conic.dart b/lib/web_ui/lib/src/engine/conic.dart
index 5de3968c7431a..f964440518909 100644
--- a/lib/web_ui/lib/src/engine/conic.dart
+++ b/lib/web_ui/lib/src/engine/conic.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Converts conic curve to a list of quadratic curves for rendering on
@@ -40,10 +40,10 @@ class Conic {
     if (subdivideCount == _maxSubdivisionCount) {
       // We have an extreme number of quads, chop this conic and check if
       // it generates a pair of lines, in which case we should not subdivide.
-      final List<Conic> dst = List<Conic>(2);
+      final _ConicPair dst = _ConicPair();
       _chop(dst);
-      final Conic conic0 = dst[0];
-      final Conic conic1 = dst[1];
+      final Conic conic0 = dst.first!;
+      final Conic conic1 = dst.second!;
       // If this chop generates pair of lines no need to subdivide.
       if (conic0.p1x == conic0.p2x &&
           conic0.p1y == conic0.p2y &&
@@ -93,10 +93,10 @@ class Conic {
       pointList.add(ui.Offset(src.p2x, src.p2y));
       return;
     }
-    final List<Conic> dst = List<Conic>(2);
+    final _ConicPair dst = _ConicPair();
     src._chop(dst);
-    final Conic conic0 = dst[0];
-    final Conic conic1 = dst[1];
+    final Conic conic0 = dst.first!;
+    final Conic conic1 = dst.second!;
     final double startY = src.p0y;
     final double endY = src.p2y;
     final double cpY = src.p1y;
@@ -135,7 +135,7 @@ class Conic {
   }
 
   // Splits conic into 2 parts based on weight.
-  void _chop(List<Conic> dst) {
+  void _chop(_ConicPair pair) {
     final double scale = 1.0 / (1.0 + fW);
     final double newW = _subdivideWeightValue(fW);
     final ui.Offset wp1 = ui.Offset(fW * p1x, fW * p1y);
@@ -147,9 +147,9 @@ class Conic {
       m = ui.Offset((p0x + (w2 * p1x) + p2x) * scaleHalf,
           (p0y + (w2 * p1y) + p2y) * scaleHalf);
     }
-    dst[0] = Conic(p0x, p0y, (p0x + wp1.dx) * scale, (p0y + wp1.dy) * scale,
+    pair.first = Conic(p0x, p0y, (p0x + wp1.dx) * scale, (p0y + wp1.dy) * scale,
         m.dx, m.dy, newW);
-    dst[1] = Conic(m.dx, m.dy, (p2x + wp1.dx) * scale, (p2y + wp1.dy) * scale,
+    pair.second = Conic(m.dx, m.dy, (p2x + wp1.dx) * scale, (p2y + wp1.dy) * scale,
         p2x, p2y, newW);
   }
 
@@ -188,3 +188,8 @@ class Conic {
     return pow2;
   }
 }
+
+class _ConicPair {
+  Conic? first;
+  Conic? second;
+}
diff --git a/lib/web_ui/lib/src/engine/dom_canvas.dart b/lib/web_ui/lib/src/engine/dom_canvas.dart
index 2ccb4b6aeb60c..1a106154314d2 100644
--- a/lib/web_ui/lib/src/engine/dom_canvas.dart
+++ b/lib/web_ui/lib/src/engine/dom_canvas.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A canvas that renders to DOM elements and CSS properties.
@@ -111,10 +111,10 @@ class DomCanvas extends EngineCanvas with SaveElementStackTracking {
       ..transform = effectiveTransform;
 
     final String cssColor =
-        paint.color == null ? '#000000' : colorToCssString(paint.color);
+        paint.color == null ? '#000000' : colorToCssString(paint.color)!;
 
     if (paint.maskFilter != null) {
-      style.filter = 'blur(${paint.maskFilter.webOnlySigma}px)';
+      style.filter = 'blur(${paint.maskFilter!.webOnlySigma}px)';
     }
 
     if (isStroke) {
@@ -179,7 +179,7 @@ class DomCanvas extends EngineCanvas with SaveElementStackTracking {
   @override
   void drawParagraph(ui.Paragraph paragraph, ui.Offset offset) {
     final html.Element paragraphElement =
-        _drawParagraphElement(paragraph, offset, transform: currentTransform);
+        _drawParagraphElement(paragraph as EngineParagraph, offset, transform: currentTransform);
     currentElement.append(paragraphElement);
   }
 
diff --git a/lib/web_ui/lib/src/engine/dom_renderer.dart b/lib/web_ui/lib/src/engine/dom_renderer.dart
index a2d03c018b176..19fefdd89f3fe 100644
--- a/lib/web_ui/lib/src/engine/dom_renderer.dart
+++ b/lib/web_ui/lib/src/engine/dom_renderer.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 class DomRenderer {
@@ -32,22 +32,22 @@ class DomRenderer {
       const html.EventStreamProvider<html.Event>('languagechange');
 
   /// Listens to window resize events.
-  StreamSubscription<html.Event> _resizeSubscription;
+  StreamSubscription<html.Event>? _resizeSubscription;
 
   /// Listens to window locale events.
-  StreamSubscription<html.Event> _localeSubscription;
+  StreamSubscription<html.Event>? _localeSubscription;
 
   /// Contains Flutter-specific CSS rules, such as default margins and
   /// paddings.
-  html.StyleElement _styleElement;
+  html.StyleElement? _styleElement;
 
   /// Configures the screen, such as scaling.
-  html.MetaElement _viewportMeta;
+  html.MetaElement? _viewportMeta;
 
   /// The canvaskit script, downloaded from a CDN. Only created if
   /// [experimentalUseSkia] is set to true.
-  html.ScriptElement get canvasKitScript => _canvasKitScript;
-  html.ScriptElement _canvasKitScript;
+  html.ScriptElement? get canvasKitScript => _canvasKitScript;
+  html.ScriptElement? _canvasKitScript;
 
   /// The element that contains the [sceneElement].
   ///
@@ -57,18 +57,18 @@ class DomRenderer {
   /// This element precedes the [glassPaneElement] so that it never receives
   /// input events. All input events are processed by [glassPaneElement] and the
   /// semantics tree.
-  html.Element get sceneHostElement => _sceneHostElement;
-  html.Element _sceneHostElement;
+  html.Element? get sceneHostElement => _sceneHostElement;
+  html.Element? _sceneHostElement;
 
   /// The last scene element rendered by the [render] method.
-  html.Element get sceneElement => _sceneElement;
-  html.Element _sceneElement;
+  html.Element? get sceneElement => _sceneElement;
+  html.Element? _sceneElement;
 
   /// This is state persistant across hot restarts that indicates what
   /// to clear.  We delay removal of old visible state to make the
   /// transition appear smooth.
   static const String _staleHotRestartStore = '__flutter_state';
-  List<html.Element> _staleHotRestartState;
+  List<html.Element?>? _staleHotRestartState;
 
   /// Used to decide if the browser tab still has the focus.
   ///
@@ -78,14 +78,14 @@ class DomRenderer {
   /// This getter calls the `hasFocus` method of the `Document` interface.
   /// See for more details:
   /// https://developer.mozilla.org/en-US/docs/Web/API/Document/hasFocus
-  bool get windowHasFocus => js_util.callMethod(html.document, 'hasFocus', <dynamic>[]);
+  bool? get windowHasFocus => js_util.callMethod(html.document, 'hasFocus', <dynamic>[]);
 
   void _setupHotRestart() {
     // This persists across hot restarts to clear stale DOM.
     _staleHotRestartState =
         js_util.getProperty(html.window, _staleHotRestartStore);
     if (_staleHotRestartState == null) {
-      _staleHotRestartState = <html.Element>[];
+      _staleHotRestartState = <html.Element?>[];
       js_util.setProperty(
           html.window, _staleHotRestartStore, _staleHotRestartState);
     }
@@ -93,7 +93,7 @@ class DomRenderer {
     registerHotRestartListener(() {
       _resizeSubscription?.cancel();
       _localeSubscription?.cancel();
-      _staleHotRestartState.addAll(<html.Element>[
+      _staleHotRestartState!.addAll(<html.Element?>[
         _glassPaneElement,
         _styleElement,
         _viewportMeta,
@@ -103,11 +103,11 @@ class DomRenderer {
   }
 
   void _clearOnHotRestart() {
-    if (_staleHotRestartState.isNotEmpty) {
-      for (html.Element element in _staleHotRestartState) {
+    if (_staleHotRestartState!.isNotEmpty) {
+      for (html.Element? element in _staleHotRestartState!) {
         element?.remove();
       }
-      _staleHotRestartState.clear();
+      _staleHotRestartState!.clear();
     }
   }
 
@@ -115,11 +115,11 @@ class DomRenderer {
   /// already in the right place, skip DOM mutation. This is both faster and
   /// more correct, because moving DOM nodes loses internal state, such as
   /// text selection.
-  void renderScene(html.Element sceneElement) {
+  void renderScene(html.Element? sceneElement) {
     if (sceneElement != _sceneElement) {
       _sceneElement?.remove();
       _sceneElement = sceneElement;
-      append(_sceneHostElement, sceneElement);
+      append(_sceneHostElement!, sceneElement!);
     }
     assert(() {
       _clearOnHotRestart();
@@ -133,38 +133,16 @@ class DomRenderer {
   /// which captures semantics input events. The semantics DOM tree must be a
   /// child of the glass pane element so that events bubble up to the glass pane
   /// if they are not handled by semantics.
-  html.Element get glassPaneElement => _glassPaneElement;
-  html.Element _glassPaneElement;
+  html.Element? get glassPaneElement => _glassPaneElement;
+  html.Element? _glassPaneElement;
 
-  final html.Element rootElement = html.document.body;
+  final html.Element rootElement = html.document.body!;
 
   void addElementClass(html.Element element, String className) {
     element.classes.add(className);
   }
 
-  void attachBeforeElement(
-      html.Element parent, html.Element before, html.Element newElement) {
-    assert(parent != null);
-    if (parent != null) {
-      assert(() {
-        if (before == null) {
-          return true;
-        }
-        if (before.parent != parent) {
-          throw Exception(
-            'attachBeforeElement was called with `before` element that\'s '
-            'not a child of the `parent` element:\n'
-            '  before: $before\n'
-            '  parent: $parent',
-          );
-        }
-        return true;
-      }());
-      parent.insertBefore(newElement, before);
-    }
-  }
-
-  html.Element createElement(String tagName, {html.Element parent}) {
+  html.Element createElement(String tagName, {html.Element? parent}) {
     final html.Element element = html.document.createElement(tagName);
     parent?.append(element);
     return element;
@@ -194,7 +172,7 @@ class DomRenderer {
     js_util.setProperty(element, name, value);
   }
 
-  void setElementStyle(html.Element element, String name, String value) {
+  void setElementStyle(html.Element element, String name, String? value) {
     if (value == null) {
       element.style.removeProperty(name);
     } else {
@@ -210,21 +188,21 @@ class DomRenderer {
     element.children.clear();
   }
 
-  html.Element getParent(html.Element element) => element.parent;
+  html.Element? getParent(html.Element element) => element.parent;
 
   void setTitle(String title) {
     html.document.title = title;
   }
 
   void setThemeColor(ui.Color color) {
-    html.MetaElement theme = html.document.querySelector('#flutterweb-theme');
+    html.MetaElement? theme = html.document.querySelector('#flutterweb-theme') as html.MetaElement?;
     if (theme == null) {
       theme = html.MetaElement()
         ..id = 'flutterweb-theme'
         ..name = 'theme-color';
-      html.document.head.append(theme);
+      html.document.head!.append(theme);
     }
-    theme.content = colorToCssString(color);
+    theme.content = colorToCssString(color)!;
   }
 
   static const String defaultFontStyle = 'normal';
@@ -237,8 +215,8 @@ class DomRenderer {
   void reset() {
     _styleElement?.remove();
     _styleElement = html.StyleElement();
-    html.document.head.append(_styleElement);
-    final html.CssStyleSheet sheet = _styleElement.sheet;
+    html.document.head!.append(_styleElement!);
+    final html.CssStyleSheet sheet = _styleElement!.sheet as html.CssStyleSheet;
     final bool isWebKit = browserEngine == BrowserEngine.webkit;
     final bool isFirefox = browserEngine == BrowserEngine.firefox;
     // TODO(butterfly): use more efficient CSS selectors; descendant selectors
@@ -329,7 +307,7 @@ flt-glass-pane * {
 ''', sheet.cssRules.length);
     }
 
-    final html.BodyElement bodyElement = html.document.body;
+    final html.BodyElement bodyElement = html.document.body!;
     setElementStyle(bodyElement, 'position', 'fixed');
     setElementStyle(bodyElement, 'top', '0');
     setElementStyle(bodyElement, 'right', '0');
@@ -361,7 +339,7 @@ flt-glass-pane * {
     bodyElement.spellcheck = false;
 
     for (html.Element viewportMeta
-        in html.document.head.querySelectorAll('meta[name="viewport"]')) {
+        in html.document.head!.querySelectorAll('meta[name="viewport"]')) {
       if (assertionsEnabled) {
         // Filter out the meta tag that we ourselves placed on the page. This is
         // to avoid UI flicker during hot restart. Hot restart will clean up the
@@ -387,26 +365,27 @@ flt-glass-pane * {
       ..name = 'viewport'
       ..content = 'width=device-width, initial-scale=1.0, '
           'maximum-scale=1.0, user-scalable=no';
-    html.document.head.append(_viewportMeta);
+    html.document.head!.append(_viewportMeta!);
 
     // IMPORTANT: the glass pane element must come after the scene element in the DOM node list so
     //            it can intercept input events.
     _glassPaneElement?.remove();
-    _glassPaneElement = createElement('flt-glass-pane');
-    _glassPaneElement.style
+    final html.Element glassPaneElement = createElement('flt-glass-pane');
+    _glassPaneElement = glassPaneElement;
+    glassPaneElement.style
       ..position = 'absolute'
       ..top = '0'
       ..right = '0'
       ..bottom = '0'
       ..left = '0';
-    bodyElement.append(_glassPaneElement);
+    bodyElement.append(glassPaneElement);
 
     _sceneHostElement = createElement('flt-scene-host');
 
     // Don't allow the scene to receive pointer events.
-    _sceneHostElement.style.pointerEvents = 'none';
+    _sceneHostElement!.style.pointerEvents = 'none';
 
-    _glassPaneElement.append(_sceneHostElement);
+    glassPaneElement.append(_sceneHostElement!);
 
     final html.Element _accesibilityPlaceholder = EngineSemanticsOwner
         .instance.semanticsHelper
@@ -420,12 +399,12 @@ flt-glass-pane * {
     glassPaneElement
         .insertBefore(_accesibilityPlaceholder, _sceneHostElement);
 
-    PointerBinding.initInstance(_glassPaneElement);
+    PointerBinding.initInstance(glassPaneElement);
 
     // Hide the DOM nodes used to render the scene from accessibility, because
     // the accessibility tree is built from the SemanticsNode tree as a parallel
     // DOM tree.
-    setElementAttribute(_sceneHostElement, 'aria-hidden', 'true');
+    setElementAttribute(_sceneHostElement!, 'aria-hidden', 'true');
 
     if (html.window.visualViewport == null && isWebKit) {
       // Safari sometimes gives us bogus innerWidth/innerHeight values when the
@@ -460,13 +439,13 @@ flt-glass-pane * {
     if (experimentalUseSkia) {
       _canvasKitScript?.remove();
       _canvasKitScript = html.ScriptElement();
-      _canvasKitScript.src = canvasKitBaseUrl + 'canvaskit.js';
-      html.document.head.append(_canvasKitScript);
+      _canvasKitScript!.src = canvasKitBaseUrl + 'canvaskit.js';
+      html.document.head!.append(_canvasKitScript!);
     }
 
     if (html.window.visualViewport != null) {
       _resizeSubscription =
-          html.window.visualViewport.onResize.listen(_metricsDidChange);
+          html.window.visualViewport!.onResize.listen(_metricsDidChange);
     } else {
       _resizeSubscription = html.window.onResize.listen(_metricsDidChange);
     }
@@ -484,7 +463,7 @@ flt-glass-pane * {
   ///
   /// Note: always check for rotations for a mobile device. Update the physical
   /// size if the change is caused by a rotation.
-  void _metricsDidChange(html.Event event) {
+  void _metricsDidChange(html.Event? event) {
     if(isMobile && !window.isRotation() && textEditing.isEditing) {
       window.computeOnScreenKeyboardInsets();
       window.invokeOnMetricsChanged();
@@ -500,7 +479,7 @@ flt-glass-pane * {
   void _languageDidChange(html.Event event) {
     window._updateLocales();
     if (ui.window.onLocaleChanged != null) {
-      ui.window.onLocaleChanged();
+      ui.window.onLocaleChanged!();
     }
   }
 
@@ -511,18 +490,18 @@ flt-glass-pane * {
   /// Removes all children of a DOM node.
   void clearDom(html.Node node) {
     while (node.lastChild != null) {
-      node.lastChild.remove();
+      node.lastChild!.remove();
     }
   }
 
-  static bool _ellipseFeatureDetected;
+  static bool? _ellipseFeatureDetected;
 
   /// Draws CanvasElement ellipse with fallback.
   static void ellipse(html.CanvasRenderingContext2D context,
       double centerX, double centerY, double radiusX, double radiusY,
       double rotation, double startAngle, double endAngle, bool antiClockwise) {
     _ellipseFeatureDetected ??= js_util.getProperty(context, 'ellipse') != null;
-    if (_ellipseFeatureDetected) {
+    if (_ellipseFeatureDetected!) {
       context.ellipse(centerX, centerY, radiusX, radiusY,
           rotation, startAngle, endAngle, antiClockwise);
     } else {
@@ -554,17 +533,17 @@ flt-glass-pane * {
   /// defer to the operating system default.
   ///
   /// See w3c screen api: https://www.w3.org/TR/screen-orientation/
-  Future<bool> setPreferredOrientation(List<dynamic> orientations) {
+  Future<bool> setPreferredOrientation(List<dynamic>? orientations) {
     final html.Screen screen = html.window.screen;
-    if (screen != null) {
+    if (!_unsafeIsNull(screen)) {
       final html.ScreenOrientation screenOrientation =
           screen.orientation;
-      if (screenOrientation != null) {
-        if (orientations.isEmpty) {
+      if (!_unsafeIsNull(screenOrientation)) {
+        if (orientations!.isEmpty) {
           screenOrientation.unlock();
           return Future.value(true);
         } else {
-          String lockType = _deviceOrientationToLockType(orientations.first);
+          String? lockType = _deviceOrientationToLockType(orientations.first);
           if (lockType != null) {
             final Completer<bool> completer = Completer<bool>();
             try {
@@ -588,7 +567,7 @@ flt-glass-pane * {
   }
 
   // Converts device orientation to w3c OrientationLockType enum.
-  static String _deviceOrientationToLockType(String deviceOrientation) {
+  static String? _deviceOrientationToLockType(String deviceOrientation) {
     switch(deviceOrientation) {
       case 'DeviceOrientation.portraitUp':
         return orientationLockTypePortraitPrimary;
@@ -604,11 +583,14 @@ flt-glass-pane * {
   }
 
   /// The element corresponding to the only child of the root surface.
-  html.Element get _rootApplicationElement {
+  html.Element? get _rootApplicationElement {
     final html.Element lastElement = rootElement.children.last;
-    return lastElement.children.singleWhere((html.Element element) {
-      return element.tagName == 'FLT-SCENE';
-    }, orElse: () => null);
+    for (html.Element child in lastElement.children) {
+      if (child.tagName == 'FLT-SCENE') {
+        return child;
+      }
+    }
+    return null;
   }
 
   /// Provides haptic feedback.
@@ -621,22 +603,22 @@ flt-glass-pane * {
 
   String get currentHtml => _rootApplicationElement?.outerHtml ?? '';
 
-  DebugDomRendererFrameStatistics _debugFrameStatistics;
+  DebugDomRendererFrameStatistics? _debugFrameStatistics;
 
-  DebugDomRendererFrameStatistics debugFlushFrameStatistics() {
+  DebugDomRendererFrameStatistics? debugFlushFrameStatistics() {
     if (!assertionsEnabled) {
       throw Exception('This code should not be reachable in production.');
     }
-    final DebugDomRendererFrameStatistics current = _debugFrameStatistics;
+    final DebugDomRendererFrameStatistics? current = _debugFrameStatistics;
     _debugFrameStatistics = DebugDomRendererFrameStatistics();
     return current;
   }
 
-  void debugRulerCacheHit() => _debugFrameStatistics.paragraphRulerCacheHits++;
+  void debugRulerCacheHit() => _debugFrameStatistics!.paragraphRulerCacheHits++;
   void debugRulerCacheMiss() =>
-      _debugFrameStatistics.paragraphRulerCacheMisses++;
-  void debugRichTextLayout() => _debugFrameStatistics.richTextLayouts++;
-  void debugPlainTextLayout() => _debugFrameStatistics.plainTextLayouts++;
+      _debugFrameStatistics!.paragraphRulerCacheMisses++;
+  void debugRichTextLayout() => _debugFrameStatistics!.richTextLayouts++;
+  void debugPlainTextLayout() => _debugFrameStatistics!.plainTextLayouts++;
 }
 
 /// Miscellaneous statistics collecting during a single frame's execution.
diff --git a/lib/web_ui/lib/src/engine/engine_canvas.dart b/lib/web_ui/lib/src/engine/engine_canvas.dart
index 1c2261e766238..cfbd885d23e6a 100644
--- a/lib/web_ui/lib/src/engine/engine_canvas.dart
+++ b/lib/web_ui/lib/src/engine/engine_canvas.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Defines canvas interface common across canvases that the [SceneBuilder]
@@ -95,19 +95,19 @@ Matrix4 transformWithOffset(Matrix4 transform, ui.Offset offset) {
 
 class _SaveStackEntry {
   _SaveStackEntry({
-    @required this.transform,
-    @required this.clipStack,
+    required this.transform,
+    required this.clipStack,
   });
 
   final Matrix4 transform;
-  final List<_SaveClipEntry> clipStack;
+  final List<_SaveClipEntry>? clipStack;
 }
 
 /// Tagged union of clipping parameters used for canvas.
 class _SaveClipEntry {
-  final ui.Rect rect;
-  final ui.RRect rrect;
-  final ui.Path path;
+  final ui.Rect? rect;
+  final ui.RRect? rrect;
+  final ui.Path? path;
   final Matrix4 currentTransform;
   _SaveClipEntry.rect(this.rect, this.currentTransform)
       : rrect = null,
@@ -129,7 +129,7 @@ mixin SaveStackTracking on EngineCanvas {
 
   /// The stack that maintains clipping operations used when text is painted
   /// onto bitmap canvas but is composited as separate element.
-  List<_SaveClipEntry> _clipStack;
+  List<_SaveClipEntry>? _clipStack;
 
   /// Returns whether there are active clipping regions on the canvas.
   bool get isClipped => _clipStack != null;
@@ -157,7 +157,7 @@ mixin SaveStackTracking on EngineCanvas {
     _saveStack.add(_SaveStackEntry(
       transform: _currentTransform.clone(),
       clipStack:
-          _clipStack == null ? null : List<_SaveClipEntry>.from(_clipStack),
+          _clipStack == null ? null : List<_SaveClipEntry>.from(_clipStack!),
     ));
   }
 
@@ -224,7 +224,7 @@ mixin SaveStackTracking on EngineCanvas {
   @override
   void clipRect(ui.Rect rect) {
     _clipStack ??= <_SaveClipEntry>[];
-    _clipStack.add(_SaveClipEntry.rect(rect, _currentTransform.clone()));
+    _clipStack!.add(_SaveClipEntry.rect(rect, _currentTransform.clone()));
   }
 
   /// Adds a round rectangle to clipping stack.
@@ -233,7 +233,7 @@ mixin SaveStackTracking on EngineCanvas {
   @override
   void clipRRect(ui.RRect rrect) {
     _clipStack ??= <_SaveClipEntry>[];
-    _clipStack.add(_SaveClipEntry.rrect(rrect, _currentTransform.clone()));
+    _clipStack!.add(_SaveClipEntry.rrect(rrect, _currentTransform.clone()));
   }
 
   /// Adds a path to clipping stack.
@@ -242,18 +242,18 @@ mixin SaveStackTracking on EngineCanvas {
   @override
   void clipPath(ui.Path path) {
     _clipStack ??= <_SaveClipEntry>[];
-    _clipStack.add(_SaveClipEntry.path(path, _currentTransform.clone()));
+    _clipStack!.add(_SaveClipEntry.path(path, _currentTransform.clone()));
   }
 }
 
 html.Element _drawParagraphElement(
   EngineParagraph paragraph,
   ui.Offset offset, {
-  Matrix4 transform,
+  Matrix4? transform,
 }) {
   assert(paragraph._isLaidOut);
 
-  final html.Element paragraphElement = paragraph._paragraphElement.clone(true);
+  final html.Element paragraphElement = paragraph._paragraphElement.clone(true) as html.Element;
 
   final html.CssStyleDeclaration paragraphStyle = paragraphElement.style;
   paragraphStyle
diff --git a/lib/web_ui/lib/src/engine/frame_reference.dart b/lib/web_ui/lib/src/engine/frame_reference.dart
index 9274d67633dce..a79829f196898 100644
--- a/lib/web_ui/lib/src/engine/frame_reference.dart
+++ b/lib/web_ui/lib/src/engine/frame_reference.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A monotonically increasing frame number being rendered.
@@ -26,7 +26,7 @@ class FrameReference<V> {
   }
 
   /// The current value of this reference.
-  V value;
+  V? value;
 }
 
 /// Cache where items cached before frame(N) is committed, can be reused in
@@ -40,16 +40,16 @@ class FrameReference<V> {
 /// at all.
 class CrossFrameCache<T> {
   // Cached items in a scene.
-  Map<String, List<_CrossFrameCacheItem<T>>> _cache;
+  Map<String, List<_CrossFrameCacheItem<T>>>? _cache;
 
   // Cached items that have been committed, ready for reuse on next frame.
-  Map<String, List<_CrossFrameCacheItem<T>>> _reusablePool;
+  Map<String, List<_CrossFrameCacheItem<T>>>? _reusablePool;
 
   // Called when a scene or picture update is committed.
   void commitFrame() {
     // Evict unused items from prior frame.
     if (_reusablePool != null) {
-      for (List<_CrossFrameCacheItem<T>> items in _reusablePool.values) {
+      for (List<_CrossFrameCacheItem<T>> items in _reusablePool!.values) {
         for (_CrossFrameCacheItem<T> item in items) {
           item.evict();
         }
@@ -64,21 +64,21 @@ class CrossFrameCache<T> {
   ///
   /// Duplicate keys are allowed. For example the same image url may be used
   /// to create multiple instances of [ImageElement] to be reused in the future.
-  void cache(String key, T value, [CrossFrameCacheEvictCallback<T> callback]) {
+  void cache(String key, T value, [CrossFrameCacheEvictCallback<T>? callback]) {
     _addToCache(key, _CrossFrameCacheItem<T>(value, callback));
   }
 
   void _addToCache(String key, _CrossFrameCacheItem<T> item) {
     _cache ??= {};
-    (_cache[key] ??= [])..add(item);
+    (_cache![key] ??= [])..add(item);
   }
 
   /// Given a key, consumes an item that has been cached in a prior frame.
-  T reuse(String key) {
+  T? reuse(String key) {
     if (_reusablePool == null) {
       return null;
     }
-    List<_CrossFrameCacheItem<T>> items = _reusablePool[key];
+    List<_CrossFrameCacheItem<T>>? items = _reusablePool![key];
     if (items == null || items.isEmpty) {
       return null;
     }
@@ -90,11 +90,11 @@ class CrossFrameCache<T> {
 
 class _CrossFrameCacheItem<T> {
   final T value;
-  final CrossFrameCacheEvictCallback<T> evictCallback;
+  final CrossFrameCacheEvictCallback<T>? evictCallback;
   _CrossFrameCacheItem(this.value, this.evictCallback);
   void evict() {
     if (evictCallback != null) {
-      evictCallback(value);
+      evictCallback!(value);
     }
   }
 }
diff --git a/lib/web_ui/lib/src/engine/history.dart b/lib/web_ui/lib/src/engine/history.dart
index 7a30d62d8f345..852dd6fa82bfc 100644
--- a/lib/web_ui/lib/src/engine/history.dart
+++ b/lib/web_ui/lib/src/engine/history.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 const MethodCall _popRouteMethodCall = MethodCall('popRoute');
@@ -34,8 +34,8 @@ bool _isFlutterEntry(dynamic state) {
 ///
 /// There should only be one global instance of this class.
 class BrowserHistory {
-  LocationStrategy _locationStrategy;
-  ui.VoidCallback _unsubscribe;
+  LocationStrategy? _locationStrategy;
+  ui.VoidCallback? _unsubscribe;
 
   /// Changing the location strategy will unsubscribe from the old strategy's
   /// event listeners, and subscribe to the new one.
@@ -45,7 +45,7 @@ class BrowserHistory {
   ///
   /// If the given strategy is null, it will render this [BrowserHistory]
   /// instance inactive.
-  set locationStrategy(LocationStrategy strategy) {
+  set locationStrategy(LocationStrategy? strategy) {
     if (strategy != _locationStrategy) {
       _tearoffStrategy(_locationStrategy);
       _locationStrategy = strategy;
@@ -55,22 +55,22 @@ class BrowserHistory {
 
   /// Returns the currently active location strategy.
   @visibleForTesting
-  LocationStrategy get locationStrategy => _locationStrategy;
+  LocationStrategy? get locationStrategy => _locationStrategy;
 
   /// The path of the current location of the user's browser.
   String get currentPath => _locationStrategy?.path ?? '/';
 
   /// Update the url with the given [routeName].
-  void setRouteName(String routeName) {
+  void setRouteName(String? routeName) {
     if (_locationStrategy != null) {
-      _setupFlutterEntry(_locationStrategy, replace: true, path: routeName);
+      _setupFlutterEntry(_locationStrategy!, replace: true, path: routeName);
     }
   }
 
   /// This method does the same thing as the browser back button.
   Future<void> back() {
     if (_locationStrategy != null) {
-      return _locationStrategy.back();
+      return _locationStrategy!.back();
     }
     return Future<void>.value();
   }
@@ -81,21 +81,21 @@ class BrowserHistory {
       _tearoffStrategy(_locationStrategy);
       // After tearing off the location strategy, we should be on the "origin"
       // entry. So we need to go back one more time to exit the app.
-      final Future<void> backFuture = _locationStrategy.back();
+      final Future<void> backFuture = _locationStrategy!.back();
       _locationStrategy = null;
       return backFuture;
     }
     return Future<void>.value();
   }
 
-  String _userProvidedRouteName;
+  String? _userProvidedRouteName;
   void _popStateListener(covariant html.PopStateEvent event) {
     if (_isOriginEntry(event.state)) {
       // If we find ourselves in the origin entry, it means that the user
       // clicked the back button.
 
       // 1. Re-push the flutter entry to keep it always at the top of history.
-      _setupFlutterEntry(_locationStrategy);
+      _setupFlutterEntry(_locationStrategy!);
 
       // 2. Send a 'popRoute' platform message so the app can handle it accordingly.
       if (window._onPlatformMessage != null) {
@@ -113,7 +113,7 @@ class BrowserHistory {
       // brings us here.
       assert(_userProvidedRouteName != null);
 
-      final String newRouteName = _userProvidedRouteName;
+      final String newRouteName = _userProvidedRouteName!;
       _userProvidedRouteName = null;
 
       // Send a 'pushRoute' platform message so the app handles it accordingly.
@@ -137,7 +137,7 @@ class BrowserHistory {
       // 2. Then we remove the new entry.
       // This will take us back to our "flutter" entry and it causes a new
       // popstate event that will be handled in the "else if" section above.
-      _locationStrategy.back();
+      _locationStrategy!.back();
     }
   }
 
@@ -145,7 +145,7 @@ class BrowserHistory {
   /// replaces the state of the entry so that we can recognize it later using
   /// [_isOriginEntry] inside [_popStateListener].
   void _setupOriginEntry(LocationStrategy strategy) {
-    assert(strategy != null);
+    assert(strategy != null); // ignore: unnecessary_null_comparison
     strategy.replaceState(_originState, 'origin', '');
   }
 
@@ -154,9 +154,9 @@ class BrowserHistory {
   void _setupFlutterEntry(
     LocationStrategy strategy, {
     bool replace = false,
-    String path,
+    String? path,
   }) {
-    assert(strategy != null);
+    assert(strategy != null); // ignore: unnecessary_null_comparison
     path ??= currentPath;
     if (replace) {
       strategy.replaceState(_flutterState, 'flutter', path);
@@ -165,7 +165,7 @@ class BrowserHistory {
     }
   }
 
-  void _setupStrategy(LocationStrategy strategy) {
+  void _setupStrategy(LocationStrategy? strategy) {
     if (strategy == null) {
       return;
     }
@@ -180,16 +180,16 @@ class BrowserHistory {
       _setupOriginEntry(strategy);
       _setupFlutterEntry(strategy, replace: false, path: path);
     }
-    _unsubscribe = strategy.onPopState(_popStateListener);
+    _unsubscribe = strategy.onPopState(_popStateListener as dynamic Function(html.Event));
   }
 
-  void _tearoffStrategy(LocationStrategy strategy) {
+  void _tearoffStrategy(LocationStrategy? strategy) {
     if (strategy == null) {
       return;
     }
 
     assert(_unsubscribe != null);
-    _unsubscribe();
+    _unsubscribe!();
     _unsubscribe = null;
 
     // Remove the "flutter" entry and go back to the "origin" entry so that the
diff --git a/lib/web_ui/lib/src/engine/houdini_canvas.dart b/lib/web_ui/lib/src/engine/houdini_canvas.dart
index dcf9e2a7c6d86..cebb845c02e60 100644
--- a/lib/web_ui/lib/src/engine/houdini_canvas.dart
+++ b/lib/web_ui/lib/src/engine/houdini_canvas.dart
@@ -7,7 +7,7 @@
 // - avoid producing DOM-based clips if there is no text
 // - evaluate using stylesheets for static CSS properties
 // - evaluate reusing houdini canvases
-// @dart = 2.6
+
 part of engine;
 
 /// A canvas that renders to a combination of HTML DOM and CSS Custom Paint API.
@@ -22,7 +22,7 @@ class HoudiniCanvas extends EngineCanvas with SaveElementStackTracking {
   /// where this canvas paints.
   ///
   /// Painting outside the bounds of this rectangle is cropped.
-  final ui.Rect bounds;
+  final ui.Rect? bounds;
 
   HoudiniCanvas(this.bounds) {
     // TODO(yjbanov): would it be faster to specify static values in a
@@ -31,8 +31,8 @@ class HoudiniCanvas extends EngineCanvas with SaveElementStackTracking {
       ..position = 'absolute'
       ..top = '0'
       ..left = '0'
-      ..width = '${bounds.size.width}px'
-      ..height = '${bounds.size.height}px'
+      ..width = '${bounds!.size.width}px'
+      ..height = '${bounds!.size.height}px'
       ..backgroundImage = 'paint(flt)';
   }
 
@@ -206,7 +206,7 @@ class HoudiniCanvas extends EngineCanvas with SaveElementStackTracking {
   void drawImageRect(
       ui.Image image, ui.Rect src, ui.Rect dst, SurfacePaintData paint) {
     // TODO(yjbanov): implement src rectangle
-    final HtmlImage htmlImage = image;
+    final HtmlImage htmlImage = image as HtmlImage;
     final html.Element imageBox = html.Element.tag('flt-img');
     final String cssTransform = matrix4ToCssTransform(
         transformWithOffset(currentTransform, ui.Offset(dst.left, dst.top)));
@@ -225,7 +225,7 @@ class HoudiniCanvas extends EngineCanvas with SaveElementStackTracking {
   @override
   void drawParagraph(ui.Paragraph paragraph, ui.Offset offset) {
     final html.Element paragraphElement =
-        _drawParagraphElement(paragraph, offset, transform: currentTransform);
+        _drawParagraphElement(paragraph as EngineParagraph, offset, transform: currentTransform);
     currentElement.append(paragraphElement);
   }
 
@@ -246,8 +246,8 @@ class HoudiniCanvas extends EngineCanvas with SaveElementStackTracking {
 
 class _SaveElementStackEntry {
   _SaveElementStackEntry({
-    @required this.savedElement,
-    @required this.transform,
+    required this.savedElement,
+    required this.transform,
   });
 
   final html.Element savedElement;
diff --git a/lib/web_ui/lib/src/engine/html_image_codec.dart b/lib/web_ui/lib/src/engine/html_image_codec.dart
index cc092307c32c8..f0279db53ac80 100644
--- a/lib/web_ui/lib/src/engine/html_image_codec.dart
+++ b/lib/web_ui/lib/src/engine/html_image_codec.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 final bool _supportsDecode = js_util.getProperty(
@@ -16,7 +16,7 @@ typedef WebOnlyImageCodecChunkCallback = void Function(
 
 class HtmlCodec implements ui.Codec {
   final String src;
-  final WebOnlyImageCodecChunkCallback/*?*/ chunkCallback;
+  final WebOnlyImageCodecChunkCallback? chunkCallback;
 
   HtmlCodec(this.src, {this.chunkCallback});
 
@@ -33,7 +33,7 @@ class HtmlCodec implements ui.Codec {
     // we add 0/100 , 100/100 progress callbacks to enable loading progress
     // builders to create UI.
     if (chunkCallback != null) {
-      chunkCallback(0, 100);
+      chunkCallback!(0, 100);
     }
     if (_supportsDecode) {
       final html.ImageElement imgElement = html.ImageElement();
@@ -41,7 +41,7 @@ class HtmlCodec implements ui.Codec {
       js_util.setProperty(imgElement, 'decoding', 'async');
       imgElement.decode().then((dynamic _) {
         if (chunkCallback != null) {
-          chunkCallback(100, 100);
+          chunkCallback!(100, 100);
         }
         final HtmlImage image = HtmlImage(
           imgElement,
@@ -62,8 +62,8 @@ class HtmlCodec implements ui.Codec {
   }
 
   void _decodeUsingOnLoad(Completer completer) {
-    StreamSubscription<html.Event> loadSubscription;
-    StreamSubscription<html.Event> errorSubscription;
+    StreamSubscription<html.Event>? loadSubscription;
+    late StreamSubscription<html.Event> errorSubscription;
     final html.ImageElement imgElement = html.ImageElement();
     // If the browser doesn't support asynchronous decoding of an image,
     // then use the `onload` event to decide when it's ready to paint to the
@@ -76,9 +76,9 @@ class HtmlCodec implements ui.Codec {
     });
     loadSubscription = imgElement.onLoad.listen((html.Event event) {
       if (chunkCallback != null) {
-        chunkCallback(100, 100);
+        chunkCallback!(100, 100);
       }
-      loadSubscription.cancel();
+      loadSubscription!.cancel();
       errorSubscription.cancel();
       final HtmlImage image = HtmlImage(
         imgElement,
@@ -133,11 +133,11 @@ class HtmlImage implements ui.Image {
   final int height;
 
   @override
-  Future<ByteData> toByteData(
+  Future<ByteData?> toByteData(
       {ui.ImageByteFormat format = ui.ImageByteFormat.rawRgba}) {
-    return futurize((Callback<ByteData> callback) {
-      return _toByteData(format.index, (Uint8List encoded) {
-        callback(encoded?.buffer?.asByteData());
+    return futurize((Callback<ByteData?> callback) {
+      return _toByteData(format.index, (Uint8List? encoded) {
+        callback(encoded?.buffer.asByteData());
       });
     });
   }
@@ -146,7 +146,7 @@ class HtmlImage implements ui.Image {
   // clones on subsequent calls.
   html.ImageElement cloneImageElement() {
     if (_requiresClone) {
-      return imgElement.clone(true);
+      return imgElement.clone(true) as html.ImageElement;
     } else {
       _requiresClone = true;
       imgElement.style.position = 'absolute';
@@ -157,7 +157,7 @@ class HtmlImage implements ui.Image {
   // TODO(het): Support this for asset images and images generated from
   // `Picture`s.
   /// Returns an error message on failure, null on success.
-  String _toByteData(int format, Callback<Uint8List> callback) {
+  String _toByteData(int format, Callback<Uint8List?> callback) {
     callback(null);
     return 'Image.toByteData is not supported in Flutter for Web';
   }
diff --git a/lib/web_ui/lib/src/engine/keyboard.dart b/lib/web_ui/lib/src/engine/keyboard.dart
index 2569a05a3ab9f..df1e5d6b66b01 100644
--- a/lib/web_ui/lib/src/engine/keyboard.dart
+++ b/lib/web_ui/lib/src/engine/keyboard.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// After a keydown is received, this is the duration we wait for a repeat event
@@ -22,8 +22,8 @@ class Keyboard {
   }
 
   /// The [Keyboard] singleton.
-  static Keyboard get instance => _instance;
-  static Keyboard _instance;
+  static Keyboard? get instance => _instance;
+  static Keyboard? _instance;
 
   /// A mapping of [KeyboardEvent.code] to [Timer].
   ///
@@ -31,8 +31,8 @@ class Keyboard {
   /// if no repeat events were received.
   final Map<String, Timer> _keydownTimers = <String, Timer>{};
 
-  html.EventListener _keydownListener;
-  html.EventListener _keyupListener;
+  html.EventListener? _keydownListener;
+  html.EventListener? _keyupListener;
 
   Keyboard._() {
     _keydownListener = (html.Event event) {
@@ -58,7 +58,7 @@ class Keyboard {
     html.window.removeEventListener('keyup', _keyupListener);
 
     for (final String key in _keydownTimers.keys) {
-      _keydownTimers[key].cancel();
+      _keydownTimers[key]!.cancel();
     }
     _keydownTimers.clear();
 
@@ -79,7 +79,7 @@ class Keyboard {
       return;
     }
 
-    final html.KeyboardEvent keyboardEvent = event as html.KeyboardEvent;
+    final html.KeyboardEvent keyboardEvent = event;
 
     if (window._onPlatformMessage == null) {
       return;
@@ -185,4 +185,4 @@ bool _isModifierKey(html.KeyboardEvent event) {
   return key == 'Meta' || key == 'Shift' || key == 'Alt' || key == 'Control';
 }
 
-void _noopCallback(ByteData data) {}
+void _noopCallback(ByteData? data) {}
diff --git a/lib/web_ui/lib/src/engine/mouse_cursor.dart b/lib/web_ui/lib/src/engine/mouse_cursor.dart
index b96f0b2b2c1cb..2ee68977e42cd 100644
--- a/lib/web_ui/lib/src/engine/mouse_cursor.dart
+++ b/lib/web_ui/lib/src/engine/mouse_cursor.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Provides mouse cursor bindings, such as the `flutter/mousecursor` channel.
@@ -15,8 +15,8 @@ class MouseCursor {
   }
 
   /// The [MouseCursor] singleton.
-  static MouseCursor get instance => _instance;
-  static MouseCursor _instance;
+  static MouseCursor? get instance => _instance;
+  static MouseCursor? _instance;
 
   MouseCursor._() {}
 
@@ -33,13 +33,13 @@ class MouseCursor {
     'horizontalDoubleArrow': 'ew-resize',
     'verticalDoubleArrow': 'ns-resize',
   };
-  static String _mapKindToCssValue(String kind) {
+  static String _mapKindToCssValue(String? kind) {
     return _kindToCssValueMap[kind] ?? 'default';
   }
 
-  void activateSystemCursor(String kind) {
+  void activateSystemCursor(String? kind) {
     domRenderer.setElementStyle(
-      domRenderer.glassPaneElement,
+      domRenderer.glassPaneElement!,
       'cursor',
       _mapKindToCssValue(kind),
     );
diff --git a/lib/web_ui/lib/src/engine/onscreen_logging.dart b/lib/web_ui/lib/src/engine/onscreen_logging.dart
index 95a39287503e3..2ec03e8e79e83 100644
--- a/lib/web_ui/lib/src/engine/onscreen_logging.dart
+++ b/lib/web_ui/lib/src/engine/onscreen_logging.dart
@@ -2,11 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
-html.Element _logElement;
-html.Element _logContainer;
+html.Element? _logElement;
+late html.Element _logContainer;
 List<_LogMessage> _logBuffer = <_LogMessage>[];
 
 class _LogMessage {
@@ -53,8 +53,8 @@ void printOnScreen(Object object) {
 
 void _initialize() {
   _logElement = html.Element.tag('flt-onscreen-log');
-  _logElement.setAttribute('aria-hidden', 'true');
-  _logElement.style
+  _logElement!.setAttribute('aria-hidden', 'true');
+  _logElement!.style
     ..position = 'fixed'
     ..left = '0'
     ..right = '0'
@@ -72,9 +72,9 @@ void _initialize() {
   _logContainer.style
     ..position = 'absolute'
     ..bottom = '0';
-  _logElement.append(_logContainer);
+  _logElement!.append(_logContainer);
 
-  html.document.body.append(_logElement);
+  html.document.body!.append(_logElement!);
 }
 
 /// Dump the current stack to the console using [print] and
@@ -86,7 +86,7 @@ void _initialize() {
 /// of lines. By default, all non-filtered stack lines are shown.
 ///
 /// The `label` argument, if present, will be printed before the stack.
-void debugPrintStack({String label, int maxFrames}) {
+void debugPrintStack({String? label, int? maxFrames}) {
   if (label != null) {
     print(label);
   }
@@ -117,11 +117,11 @@ Iterable<String> defaultStackFilter(Iterable<String> frames) {
   final List<String> result = <String>[];
   final List<String> skipped = <String>[];
   for (String line in frames) {
-    final Match match = stackParser.firstMatch(line);
+    final Match? match = stackParser.firstMatch(line);
     if (match != null) {
       assert(match.groupCount == 2);
       if (filteredPackages.contains(match.group(2))) {
-        final Match packageMatch = packageParser.firstMatch(match.group(2));
+        final Match? packageMatch = packageParser.firstMatch(match.group(2)!);
         if (packageMatch != null && packageMatch.group(1) == 'package') {
           skipped.add(
               'package ${packageMatch.group(2)}'); // avoid "package package:foo"
@@ -149,6 +149,6 @@ Iterable<String> defaultStackFilter(Iterable<String> frames) {
   return result;
 }
 
-String debugIdentify(Object object) {
-  return '${object.runtimeType}(@${object.hashCode})';
+String debugIdentify(Object? object) {
+  return '${object!.runtimeType}(@${object.hashCode})';
 }
diff --git a/lib/web_ui/lib/src/engine/path_to_svg.dart b/lib/web_ui/lib/src/engine/path_to_svg.dart
index be73c1e3911a0..d3f07ca47a8bc 100644
--- a/lib/web_ui/lib/src/engine/path_to_svg.dart
+++ b/lib/web_ui/lib/src/engine/path_to_svg.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Converts [path] to SVG path syntax to be used as "d" attribute in path
@@ -13,20 +13,20 @@ void pathToSvg(SurfacePath path, StringBuffer sb,
     for (PathCommand command in subPath.commands) {
       switch (command.type) {
         case PathCommandTypes.moveTo:
-          final MoveTo moveTo = command;
+          final MoveTo moveTo = command as MoveTo;
           sb.write('M ${moveTo.x + offsetX} ${moveTo.y + offsetY}');
           break;
         case PathCommandTypes.lineTo:
-          final LineTo lineTo = command;
+          final LineTo lineTo = command as LineTo;
           sb.write('L ${lineTo.x + offsetX} ${lineTo.y + offsetY}');
           break;
         case PathCommandTypes.bezierCurveTo:
-          final BezierCurveTo curve = command;
+          final BezierCurveTo curve = command as BezierCurveTo;
           sb.write('C ${curve.x1 + offsetX} ${curve.y1 + offsetY} '
               '${curve.x2 + offsetX} ${curve.y2 + offsetY} ${curve.x3 + offsetX} ${curve.y3 + offsetY}');
           break;
         case PathCommandTypes.quadraticCurveTo:
-          final QuadraticCurveTo quadraticCurveTo = command;
+          final QuadraticCurveTo quadraticCurveTo = command as QuadraticCurveTo;
           sb.write(
               'Q ${quadraticCurveTo.x1 + offsetX} ${quadraticCurveTo.y1 + offsetY} '
               '${quadraticCurveTo.x2 + offsetX} ${quadraticCurveTo.y2 + offsetY}');
@@ -35,7 +35,7 @@ void pathToSvg(SurfacePath path, StringBuffer sb,
           sb.write('Z');
           break;
         case PathCommandTypes.ellipse:
-          final Ellipse ellipse = command;
+          final Ellipse ellipse = command as Ellipse;
           // Handle edge case where start and end points are the same by drawing
           // 2 half arcs.
           if ((ellipse.endAngle - ellipse.startAngle) % (2 * math.pi) == 0.0) {
@@ -74,7 +74,7 @@ void pathToSvg(SurfacePath path, StringBuffer sb,
           }
           break;
         case PathCommandTypes.rRect:
-          final RRectCommand rrectCommand = command;
+          final RRectCommand rrectCommand = command as RRectCommand;
           final ui.RRect rrect = rrectCommand.rrect;
           double left = rrect.left + offsetX;
           double right = rrect.right + offsetX;
@@ -125,7 +125,7 @@ void pathToSvg(SurfacePath path, StringBuffer sb,
           );
           break;
         case PathCommandTypes.rect:
-          final RectCommand rectCommand = command;
+          final RectCommand rectCommand = command as RectCommand;
           final bool horizontalSwap = rectCommand.width < 0;
           final double left = offsetX +
               (horizontalSwap
diff --git a/lib/web_ui/lib/src/engine/picture.dart b/lib/web_ui/lib/src/engine/picture.dart
index 04010a28f27f4..62dadf917e4f3 100644
--- a/lib/web_ui/lib/src/engine/picture.dart
+++ b/lib/web_ui/lib/src/engine/picture.dart
@@ -2,23 +2,22 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// An implementation of [ui.PictureRecorder] backed by a [RecordingCanvas].
 class EnginePictureRecorder implements ui.PictureRecorder {
   EnginePictureRecorder();
 
-  RecordingCanvas _canvas;
-  ui.Rect cullRect;
+  RecordingCanvas? _canvas;
+  late ui.Rect cullRect;
   bool _isRecording = false;
 
   RecordingCanvas beginRecording(ui.Rect bounds) {
     assert(!_isRecording);
     cullRect = bounds;
     _isRecording = true;
-    _canvas = RecordingCanvas(cullRect);
-    return _canvas;
+    return _canvas = RecordingCanvas(cullRect);
   }
 
   @override
@@ -26,13 +25,13 @@ class EnginePictureRecorder implements ui.PictureRecorder {
 
   @override
   ui.Picture endRecording() {
-    // Returning null is what the flutter engine does:
-    // lib/ui/painting/picture_recorder.cc
     if (!_isRecording) {
-      return null;
+      // The mobile version returns an empty picture in this case. To match the
+      // behavior we produce a blank picture too.
+      beginRecording(ui.Rect.largest);
     }
     _isRecording = false;
-    _canvas.endRecording();
+    _canvas!.endRecording();
     return EnginePicture(_canvas, cullRect);
   }
 }
@@ -49,7 +48,7 @@ class EnginePicture implements ui.Picture {
   Future<ui.Image> toImage(int width, int height) async {
     final ui.Rect imageRect = ui.Rect.fromLTRB(0, 0, width.toDouble(), height.toDouble());
     final BitmapCanvas canvas = BitmapCanvas(imageRect);
-    recordingCanvas.apply(canvas, imageRect);
+    recordingCanvas!.apply(canvas, imageRect);
     final String imageDataUrl = canvas.toDataUrl();
     final html.ImageElement imageElement = html.ImageElement()
       ..src = imageDataUrl
@@ -68,6 +67,6 @@ class EnginePicture implements ui.Picture {
   @override
   int get approximateBytesUsed => 0;
 
-  final RecordingCanvas recordingCanvas;
-  final ui.Rect cullRect;
+  final RecordingCanvas? recordingCanvas;
+  final ui.Rect? cullRect;
 }
diff --git a/lib/web_ui/lib/src/engine/platform_views.dart b/lib/web_ui/lib/src/engine/platform_views.dart
index 1d686da046b3f..813baa6edc92e 100644
--- a/lib/web_ui/lib/src/engine/platform_views.dart
+++ b/lib/web_ui/lib/src/engine/platform_views.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 // TODO(yjbanov): The code in this file was temporarily moved to lib/web_ui/lib/ui.dart
diff --git a/lib/web_ui/lib/src/engine/plugins.dart b/lib/web_ui/lib/src/engine/plugins.dart
index b419ecab4e419..5998d8a5c4b69 100644
--- a/lib/web_ui/lib/src/engine/plugins.dart
+++ b/lib/web_ui/lib/src/engine/plugins.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
-Future<void> Function(String, ByteData, ui.PlatformMessageResponseCallback) pluginMessageCallHandler;
+Future<void> Function(String, ByteData?, ui.PlatformMessageResponseCallback?)? pluginMessageCallHandler;
diff --git a/lib/web_ui/lib/src/engine/pointer_binding.dart b/lib/web_ui/lib/src/engine/pointer_binding.dart
index 48e43dc72db45..5bb8e41cc723e 100644
--- a/lib/web_ui/lib/src/engine/pointer_binding.dart
+++ b/lib/web_ui/lib/src/engine/pointer_binding.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Set this flag to true to see all the fired events in the console.
@@ -54,33 +54,33 @@ int convertButtonToButtons(int button) {
 
 class PointerBinding {
   /// The singleton instance of this object.
-  static PointerBinding get instance => _instance;
-  static PointerBinding _instance;
+  static PointerBinding? get instance => _instance;
+  static PointerBinding? _instance;
 
   static void initInstance(html.Element glassPaneElement) {
     if (_instance == null) {
       _instance = PointerBinding._(glassPaneElement);
       assert(() {
         registerHotRestartListener(() {
-          _instance._adapter?.clearListeners();
-          _instance._pointerDataConverter?.clearPointerState();
+          _instance!._adapter.clearListeners();
+          _instance!._pointerDataConverter.clearPointerState();
         });
         return true;
       }());
     }
   }
 
-  PointerBinding._(this.glassPaneElement) {
-    _pointerDataConverter = PointerDataConverter();
-    _detector = const PointerSupportDetector();
+  PointerBinding._(this.glassPaneElement)
+    : _pointerDataConverter = PointerDataConverter(),
+      _detector = const PointerSupportDetector() {
     _adapter = _createAdapter();
   }
 
   final html.Element glassPaneElement;
 
   PointerSupportDetector _detector;
-  _BaseAdapter _adapter;
   PointerDataConverter _pointerDataConverter;
+  late _BaseAdapter _adapter;
 
   /// Should be used in tests to define custom detection of pointer support.
   ///
@@ -99,14 +99,14 @@ class PointerBinding {
   ///
   /// PointerBinding.instance.debugOverrideDetector(MyTestDetector());
   /// ```
-  void debugOverrideDetector(PointerSupportDetector newDetector) {
+  void debugOverrideDetector(PointerSupportDetector? newDetector) {
     newDetector ??= const PointerSupportDetector();
     // When changing the detector, we need to swap the adapter.
     if (newDetector != _detector) {
       _detector = newDetector;
-      _adapter?.clearListeners();
+      _adapter.clearListeners();
       _adapter = _createAdapter();
-      _pointerDataConverter?.clearPointerState();
+      _pointerDataConverter.clearPointerState();
     }
   }
 
@@ -120,7 +120,7 @@ class PointerBinding {
     if (_detector.hasMouseEvents) {
       return _MouseAdapter(_onPointerData, glassPaneElement, _pointerDataConverter);
     }
-    return null;
+    throw UnsupportedError('This browser does not support pointer, touch, or mouse events.');
   }
 
   void _onPointerData(Iterable<ui.PointerData> data) {
@@ -196,7 +196,7 @@ abstract class _BaseAdapter {
     bool acceptOutsideGlasspane = false,
   }) {
     final html.EventListener loggedHandler = (html.Event event) {
-      if (!acceptOutsideGlasspane && !glassPaneElement.contains(event.target)) {
+      if (!acceptOutsideGlasspane && !glassPaneElement.contains(event.target as html.Node?)) {
         return;
       }
 
@@ -238,8 +238,8 @@ mixin _WheelEventListenerMixin on _BaseAdapter {
 
     // Flutter only supports pixel scroll delta. Convert deltaMode values
     // to pixels.
-    double deltaX = event.deltaX;
-    double deltaY = event.deltaY;
+    double deltaX = event.deltaX as double;
+    double deltaY = event.deltaY as double;
     switch (event.deltaMode) {
       case domDeltaLine:
         deltaX *= 32.0;
@@ -261,8 +261,8 @@ mixin _WheelEventListenerMixin on _BaseAdapter {
       kind: ui.PointerDeviceKind.mouse,
       signalKind: ui.PointerSignalKind.scroll,
       device: _mouseDeviceId,
-      physicalX: event.client.x * ui.window.devicePixelRatio,
-      physicalY: event.client.y * ui.window.devicePixelRatio,
+      physicalX: event.client.x * ui.window.devicePixelRatio as double,
+      physicalY: event.client.y * ui.window.devicePixelRatio as double,
       buttons: event.buttons,
       pressure: 1.0,
       pressureMin: 0.0,
@@ -292,8 +292,8 @@ mixin _WheelEventListenerMixin on _BaseAdapter {
 @immutable
 class _SanitizedDetails {
   const _SanitizedDetails({
-    @required this.buttons,
-    @required this.change,
+    required this.buttons,
+    required this.change,
   });
 
   final ui.PointerChange change;
@@ -326,8 +326,8 @@ class _ButtonSanitizer {
   }
 
   _SanitizedDetails sanitizeDownEvent({
-    @required int button,
-    @required int buttons,
+    required int button,
+    required int buttons,
   }) {
     // If the pointer is already down, we just send a move event with the new
     // `buttons` value.
@@ -342,7 +342,7 @@ class _ButtonSanitizer {
     );
   }
 
-  _SanitizedDetails sanitizeMoveEvent({@required int buttons}) {
+  _SanitizedDetails sanitizeMoveEvent({required int buttons}) {
     final int newPressedButtons = _htmlButtonsToFlutterButtons(buttons);
     // This could happen when the context menu is active and the user clicks
     // RMB somewhere else. The browser sends a down event with `buttons:0`.
@@ -375,7 +375,7 @@ class _ButtonSanitizer {
     );
   }
 
-  _SanitizedDetails sanitizeUpEvent() {
+  _SanitizedDetails? sanitizeUpEvent() {
     // The pointer could have been released by a `pointerout` event, in which
     // case `pointerup` should have no effect.
     if (_pressedButtons == 0) {
@@ -406,8 +406,8 @@ class _PointerAdapter extends _BaseAdapter with _WheelEventListenerMixin {
   _PointerAdapter(
     _PointerDataCallback callback,
     html.Element glassPaneElement,
-    PointerDataConverter _pointerDataConverter
-  ) : super(callback, glassPaneElement, _pointerDataConverter);
+    PointerDataConverter pointerDataConverter
+  ) : super(callback, glassPaneElement, pointerDataConverter);
 
   final Map<int, _ButtonSanitizer> _sanitizers = <int, _ButtonSanitizer>{};
 
@@ -419,8 +419,8 @@ class _PointerAdapter extends _BaseAdapter with _WheelEventListenerMixin {
   }
 
   _ButtonSanitizer _getSanitizer(int device) {
-    final _ButtonSanitizer sanitizer = _sanitizers[device];
-    assert(sanitizer != null);
+    final _ButtonSanitizer sanitizer = _sanitizers[device]!;
+    assert(sanitizer != null); // ignore: unnecessary_null_comparison
     return sanitizer;
   }
 
@@ -436,7 +436,7 @@ class _PointerAdapter extends _BaseAdapter with _WheelEventListenerMixin {
     bool acceptOutsideGlasspane = false,
   }) {
     addEventListener(eventName, (html.Event event) {
-      final html.PointerEvent pointerEvent = event;
+      final html.PointerEvent pointerEvent = event as html.PointerEvent;
       return handler(pointerEvent);
     }, acceptOutsideGlasspane: acceptOutsideGlasspane);
   }
@@ -471,7 +471,7 @@ class _PointerAdapter extends _BaseAdapter with _WheelEventListenerMixin {
     _addPointerEventListener('pointerup', (html.PointerEvent event) {
       final int device = event.pointerId;
       final List<ui.PointerData> pointerData = <ui.PointerData>[];
-      final _SanitizedDetails details = _getSanitizer(device).sanitizeUpEvent();
+      final _SanitizedDetails? details = _getSanitizer(device).sanitizeUpEvent();
       _removePointerIfUnhoverable(event);
       if (details != null) {
         _convertEventsToPointerData(data: pointerData, event: event, details: details);
@@ -495,7 +495,7 @@ class _PointerAdapter extends _BaseAdapter with _WheelEventListenerMixin {
       if (_debugLogPointerEvents) {
         print(event.type);
       }
-      _callback(_convertWheelEventToPointerData(event));
+      _callback(_convertWheelEventToPointerData(event as html.WheelEvent));
       // Prevent default so mouse wheel event doesn't get converted to
       // a scroll event that semantic nodes would process.
       event.preventDefault();
@@ -505,13 +505,13 @@ class _PointerAdapter extends _BaseAdapter with _WheelEventListenerMixin {
   // For each event that is de-coalesced from `event` and described in
   // `details`, convert it to pointer data and store in `data`.
   void _convertEventsToPointerData({
-    @required List<ui.PointerData> data,
-    @required html.PointerEvent event,
-    @required _SanitizedDetails details,
+    required List<ui.PointerData> data,
+    required html.PointerEvent event,
+    required _SanitizedDetails details,
   }) {
-    assert(data != null);
-    assert(event != null);
-    assert(details != null);
+    assert(data != null); // ignore: unnecessary_null_comparison
+    assert(event != null); // ignore: unnecessary_null_comparison
+    assert(details != null); // ignore: unnecessary_null_comparison
     final ui.PointerDeviceKind kind = _pointerTypeToDeviceKind(event.pointerType);
     // We force `device: _mouseDeviceId` on mouse pointers because Wheel events
     // might come before any PointerEvents, and since wheel events don't contain
@@ -526,10 +526,10 @@ class _PointerAdapter extends _BaseAdapter with _WheelEventListenerMixin {
       kind: kind,
       signalKind: ui.PointerSignalKind.none,
       device: device,
-      physicalX: event.client.x * ui.window.devicePixelRatio,
-      physicalY: event.client.y * ui.window.devicePixelRatio,
+      physicalX: event.client.x * ui.window.devicePixelRatio as double,
+      physicalY: event.client.y * ui.window.devicePixelRatio as double,
       buttons: details.buttons,
-      pressure: event.pressure,
+      pressure: event.pressure as double,
       pressureMin: 0.0,
       pressureMax: 1.0,
       tilt: tilt,
@@ -578,8 +578,8 @@ class _TouchAdapter extends _BaseAdapter {
   _TouchAdapter(
     _PointerDataCallback callback,
     html.Element glassPaneElement,
-    PointerDataConverter _pointerDataConverter
-  ) : super(callback, glassPaneElement, _pointerDataConverter);
+    PointerDataConverter pointerDataConverter
+  ) : super(callback, glassPaneElement, pointerDataConverter);
 
   final Set<int> _pressedTouches = <int>{};
   bool _isTouchPressed(int identifier) => _pressedTouches.contains(identifier);
@@ -588,7 +588,7 @@ class _TouchAdapter extends _BaseAdapter {
 
   void _addTouchEventListener(String eventName, _TouchEventListener handler) {
     addEventListener(eventName, (html.Event event) {
-      final html.TouchEvent touchEvent = event;
+      final html.TouchEvent touchEvent = event as html.TouchEvent;
       return handler(touchEvent);
     });
   }
@@ -676,11 +676,11 @@ class _TouchAdapter extends _BaseAdapter {
   }
 
   void _convertEventToPointerData({
-    @required List<ui.PointerData> data,
-    @required ui.PointerChange change,
-    @required html.Touch touch,
-    @required bool pressed,
-    @required Duration timeStamp,
+    required List<ui.PointerData> data,
+    required ui.PointerChange change,
+    required html.Touch touch,
+    required bool pressed,
+    required Duration timeStamp,
   }) {
     _pointerDataConverter.convert(
       data,
@@ -689,8 +689,8 @@ class _TouchAdapter extends _BaseAdapter {
       kind: ui.PointerDeviceKind.touch,
       signalKind: ui.PointerSignalKind.none,
       device: touch.identifier,
-      physicalX: touch.client.x * ui.window.devicePixelRatio,
-      physicalY: touch.client.y * ui.window.devicePixelRatio,
+      physicalX: touch.client.x * ui.window.devicePixelRatio as double,
+      physicalY: touch.client.y * ui.window.devicePixelRatio as double,
       buttons: pressed ? _kPrimaryMouseButton : 0,
       pressure: 1.0,
       pressureMin: 0.0,
@@ -723,8 +723,8 @@ class _MouseAdapter extends _BaseAdapter with _WheelEventListenerMixin {
   _MouseAdapter(
     _PointerDataCallback callback,
     html.Element glassPaneElement,
-    PointerDataConverter _pointerDataConverter
-  ) : super(callback, glassPaneElement, _pointerDataConverter);
+    PointerDataConverter pointerDataConverter
+  ) : super(callback, glassPaneElement, pointerDataConverter);
 
   final _ButtonSanitizer _sanitizer = _ButtonSanitizer();
 
@@ -734,7 +734,7 @@ class _MouseAdapter extends _BaseAdapter with _WheelEventListenerMixin {
     bool acceptOutsideGlasspane = false,
   }) {
     addEventListener(eventName, (html.Event event) {
-      final html.MouseEvent mouseEvent = event;
+      final html.MouseEvent mouseEvent = event as html.MouseEvent;
       return handler(mouseEvent);
     }, acceptOutsideGlasspane: acceptOutsideGlasspane);
   }
@@ -763,7 +763,7 @@ class _MouseAdapter extends _BaseAdapter with _WheelEventListenerMixin {
       final List<ui.PointerData> pointerData = <ui.PointerData>[];
       final bool isEndOfDrag = event.buttons == 0;
       final _SanitizedDetails sanitizedDetails = isEndOfDrag ?
-        _sanitizer.sanitizeUpEvent() :
+        _sanitizer.sanitizeUpEvent()! :
         _sanitizer.sanitizeMoveEvent(buttons: event.buttons);
       _convertEventsToPointerData(data: pointerData, event: event, details: sanitizedDetails);
       _callback(pointerData);
@@ -774,7 +774,7 @@ class _MouseAdapter extends _BaseAdapter with _WheelEventListenerMixin {
       if (_debugLogPointerEvents) {
         print(event.type);
       }
-      _callback(_convertWheelEventToPointerData(event));
+      _callback(_convertWheelEventToPointerData(event as html.WheelEvent));
       // Prevent default so mouse wheel event doesn't get converted to
       // a scroll event that semantic nodes would process.
       event.preventDefault();
@@ -784,13 +784,13 @@ class _MouseAdapter extends _BaseAdapter with _WheelEventListenerMixin {
   // For each event that is de-coalesced from `event` and described in
   // `detailsList`, convert it to pointer data and store in `data`.
   void _convertEventsToPointerData({
-    @required List<ui.PointerData> data,
-    @required html.MouseEvent event,
-    @required _SanitizedDetails details,
+    required List<ui.PointerData> data,
+    required html.MouseEvent event,
+    required _SanitizedDetails details,
   }) {
-    assert(data != null);
-    assert(event != null);
-    assert(details != null);
+    assert(data != null); // ignore: unnecessary_null_comparison
+    assert(event != null); // ignore: unnecessary_null_comparison
+    assert(details != null); // ignore: unnecessary_null_comparison
     _pointerDataConverter.convert(
       data,
       change: details.change,
@@ -798,8 +798,8 @@ class _MouseAdapter extends _BaseAdapter with _WheelEventListenerMixin {
       kind: ui.PointerDeviceKind.mouse,
       signalKind: ui.PointerSignalKind.none,
       device: _mouseDeviceId,
-      physicalX: event.client.x * ui.window.devicePixelRatio,
-      physicalY: event.client.y * ui.window.devicePixelRatio,
+      physicalX: event.client.x * ui.window.devicePixelRatio as double,
+      physicalY: event.client.y * ui.window.devicePixelRatio as double,
       buttons: details.buttons,
       pressure: 1.0,
       pressureMin: 0.0,
diff --git a/lib/web_ui/lib/src/engine/pointer_converter.dart b/lib/web_ui/lib/src/engine/pointer_converter.dart
index f08ef76c0c937..a1e47d473a426 100644
--- a/lib/web_ui/lib/src/engine/pointer_converter.dart
+++ b/lib/web_ui/lib/src/engine/pointer_converter.dart
@@ -2,15 +2,15 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 class _PointerState {
   _PointerState(this.x, this.y);
 
   /// The identifier used in framework hit test.
-  int get pointer => _pointer;
-  int _pointer;
+  int? get pointer => _pointer;
+  int? _pointer;
   static int _pointerCount = 0;
   void startNewPointer() {
     _pointerCount += 1;
@@ -65,33 +65,33 @@ class PointerDataConverter {
   }
 
   ui.PointerData _generateCompletePointerData({
-    Duration timeStamp,
-    ui.PointerChange change,
-    ui.PointerDeviceKind kind,
-    ui.PointerSignalKind signalKind,
-    int device,
-    double physicalX,
-    double physicalY,
-    int buttons,
-    bool obscured,
-    double pressure,
-    double pressureMin,
-    double pressureMax,
-    double distance,
-    double distanceMax,
-    double size,
-    double radiusMajor,
-    double radiusMinor,
-    double radiusMin,
-    double radiusMax,
-    double orientation,
-    double tilt,
-    int platformData,
-    double scrollDeltaX,
-    double scrollDeltaY,
+    required Duration timeStamp,
+    required ui.PointerChange change,
+    required ui.PointerDeviceKind kind,
+    ui.PointerSignalKind? signalKind,
+    required int device,
+    required double physicalX,
+    required double physicalY,
+    required int buttons,
+    required bool obscured,
+    required double pressure,
+    required double pressureMin,
+    required double pressureMax,
+    required double distance,
+    required double distanceMax,
+    required double size,
+    required double radiusMajor,
+    required double radiusMinor,
+    required double radiusMin,
+    required double radiusMax,
+    required double orientation,
+    required double tilt,
+    required int platformData,
+    required double scrollDeltaX,
+    required double scrollDeltaY,
   }) {
     assert(_pointers.containsKey(device));
-    final _PointerState state = _pointers[device];
+    final _PointerState state = _pointers[device]!;
     final double deltaX = physicalX - state.x;
     final double deltaY = physicalY - state.y;
     state.x = physicalX;
@@ -129,37 +129,37 @@ class PointerDataConverter {
 
   bool _locationHasChanged(int device, double physicalX, double physicalY) {
     assert(_pointers.containsKey(device));
-    final _PointerState state = _pointers[device];
+    final _PointerState state = _pointers[device]!;
     return state.x != physicalX || state.y != physicalY;
   }
 
   ui.PointerData _synthesizePointerData({
-    Duration timeStamp,
-    ui.PointerChange change,
-    ui.PointerDeviceKind kind,
-    int device,
-    double physicalX,
-    double physicalY,
-    int buttons,
-    bool obscured,
-    double pressure,
-    double pressureMin,
-    double pressureMax,
-    double distance,
-    double distanceMax,
-    double size,
-    double radiusMajor,
-    double radiusMinor,
-    double radiusMin,
-    double radiusMax,
-    double orientation,
-    double tilt,
-    int platformData,
-    double scrollDeltaX,
-    double scrollDeltaY,
+    required Duration timeStamp,
+    required ui.PointerChange change,
+    required ui.PointerDeviceKind kind,
+    required int device,
+    required double physicalX,
+    required double physicalY,
+    required int buttons,
+    required bool obscured,
+    required double pressure,
+    required double pressureMin,
+    required double pressureMax,
+    required double distance,
+    required double distanceMax,
+    required double size,
+    required double radiusMajor,
+    required double radiusMinor,
+    required double radiusMin,
+    required double radiusMax,
+    required double orientation,
+    required double tilt,
+    required int platformData,
+    required double scrollDeltaX,
+    required double scrollDeltaY,
   }) {
     assert(_pointers.containsKey(device));
-    final _PointerState state = _pointers[device];
+    final _PointerState state = _pointers[device]!;
     final double deltaX = physicalX - state.x;
     final double deltaY = physicalY - state.y;
     state.x = physicalX;
@@ -206,7 +206,7 @@ class PointerDataConverter {
     Duration timeStamp = Duration.zero,
     ui.PointerChange change = ui.PointerChange.cancel,
     ui.PointerDeviceKind kind = ui.PointerDeviceKind.touch,
-    ui.PointerSignalKind signalKind,
+    ui.PointerSignalKind? signalKind,
     int device = 0,
     double physicalX = 0.0,
     double physicalY = 0.0,
@@ -228,7 +228,7 @@ class PointerDataConverter {
     double scrollDeltaX = 0.0,
     double scrollDeltaY = 0.0,
   }) {
-    assert(change != null);
+    assert(change != null); // ignore: unnecessary_null_comparison
     if (signalKind == null ||
       signalKind == ui.PointerSignalKind.none) {
       switch (change) {
@@ -429,7 +429,7 @@ class PointerDataConverter {
           break;
         case ui.PointerChange.move:
           assert(_pointers.containsKey(device));
-          final _PointerState state = _pointers[device];
+          final _PointerState state = _pointers[device]!;
           assert(state.down);
           result.add(
             _generateCompletePointerData(
@@ -463,7 +463,7 @@ class PointerDataConverter {
         case ui.PointerChange.up:
         case ui.PointerChange.cancel:
           assert(_pointers.containsKey(device));
-          final _PointerState state = _pointers[device];
+          final _PointerState state = _pointers[device]!;
           assert(state.down);
           // Cancel events can have different coordinates due to various
           // reasons (window lost focus which is accompanied by window
@@ -570,7 +570,7 @@ class PointerDataConverter {
           break;
         case ui.PointerChange.remove:
           assert(_pointers.containsKey(device));
-          final _PointerState state = _pointers[device];
+          final _PointerState state = _pointers[device]!;
           assert(!state.down);
           result.add(
             _generateCompletePointerData(
diff --git a/lib/web_ui/lib/src/engine/profiler.dart b/lib/web_ui/lib/src/engine/profiler.dart
index 7e98bdd06e1ac..811d0a89b6a37 100644
--- a/lib/web_ui/lib/src/engine/profiler.dart
+++ b/lib/web_ui/lib/src/engine/profiler.dart
@@ -2,11 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A function that receives a benchmark [value] labeleb by [name].
-typedef OnBenchmark = void Function(String name, num value);
+typedef OnBenchmark = void Function(String name, double value);
 
 /// A function that computes a value of type [R].
 ///
@@ -38,7 +38,7 @@ R timeAction<R>(String name, Action<R> action) {
     final Stopwatch stopwatch = Stopwatch()..start();
     final R result = action();
     stopwatch.stop();
-    Profiler.instance.benchmark(name, stopwatch.elapsedMicroseconds);
+    Profiler.instance.benchmark(name, stopwatch.elapsedMicroseconds.toDouble());
     return result;
   }
 }
@@ -73,17 +73,18 @@ class Profiler {
 
   static Profiler get instance {
     _checkBenchmarkMode();
-    if (_instance == null) {
+    final Profiler? profiler = _instance;
+    if (profiler == null) {
       throw Exception(
         'Profiler has not been properly initialized. '
         'Make sure Profiler.ensureInitialized() is being called before you '
         'access Profiler.instance',
       );
     }
-    return _instance;
+    return profiler;
   }
 
-  static Profiler _instance;
+  static Profiler? _instance;
 
   static void _checkBenchmarkMode() {
     if (!isBenchmarkMode) {
@@ -96,10 +97,10 @@ class Profiler {
   }
 
   /// Used to send benchmark data to whoever is listening to them.
-  void benchmark(String name, num value) {
+  void benchmark(String name, double value) {
     _checkBenchmarkMode();
 
-    final OnBenchmark onBenchmark =
+    final OnBenchmark? onBenchmark =
         js_util.getProperty(html.window, '_flutter_internal_on_benchmark');
     if (onBenchmark != null) {
       onBenchmark(name, value);
diff --git a/lib/web_ui/lib/src/engine/rrect_renderer.dart b/lib/web_ui/lib/src/engine/rrect_renderer.dart
index 79e2377706042..3585d7c5fcf73 100644
--- a/lib/web_ui/lib/src/engine/rrect_renderer.dart
+++ b/lib/web_ui/lib/src/engine/rrect_renderer.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Renders an RRect using path primitives.
@@ -231,19 +231,19 @@ typedef RRectRendererCallback = void Function(double x, double y);
 class RRectMetricsRenderer extends _RRectRenderer {
   RRectMetricsRenderer({this.moveToCallback, this.lineToCallback, this.ellipseCallback});
 
-  final RRectRendererEllipseCallback ellipseCallback;
-  final RRectRendererCallback lineToCallback;
-  final RRectRendererCallback moveToCallback;
+  final RRectRendererEllipseCallback? ellipseCallback;
+  final RRectRendererCallback? lineToCallback;
+  final RRectRendererCallback? moveToCallback;
   @override
   void beginPath() {}
 
   @override
-  void ellipse(double centerX, double centerY, double radiusX, double radiusY, double rotation, double startAngle, double endAngle, bool antiClockwise) => ellipseCallback(
+  void ellipse(double centerX, double centerY, double radiusX, double radiusY, double rotation, double startAngle, double endAngle, bool antiClockwise) => ellipseCallback!(
       centerX, centerY, radiusX, radiusY, rotation, startAngle, endAngle, antiClockwise);
 
   @override
-  void lineTo(double x, double y) => lineToCallback(x, y);
+  void lineTo(double x, double y) => lineToCallback!(x, y);
 
   @override
-  void moveTo(double x, double y) => moveToCallback(x, y);
+  void moveTo(double x, double y) => moveToCallback!(x, y);
 }
diff --git a/lib/web_ui/lib/src/engine/semantics/accessibility.dart b/lib/web_ui/lib/src/engine/semantics/accessibility.dart
index 2d874f8cf01da..bc17ef958446c 100644
--- a/lib/web_ui/lib/src/engine/semantics/accessibility.dart
+++ b/lib/web_ui/lib/src/engine/semantics/accessibility.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Singleton for accessing accessibility announcements from the platform.
@@ -18,7 +18,7 @@ class AccessibilityAnnouncements {
     return _instance ??= AccessibilityAnnouncements._();
   }
 
-  static AccessibilityAnnouncements _instance;
+  static AccessibilityAnnouncements? _instance;
 
   AccessibilityAnnouncements._() {
     registerHotRestartListener(() {
@@ -31,7 +31,7 @@ class AccessibilityAnnouncements {
   ///
   /// The element is added to the DOM temporarily for announcing the
   /// message to the assistive technology.
-  Timer _removeElementTimer;
+  Timer? _removeElementTimer;
 
   /// The duration the accessibility announcements stay on the DOM.
   ///
@@ -47,20 +47,20 @@ class AccessibilityAnnouncements {
   /// This element has aria-live attribute.
   ///
   /// It also has id 'accessibility-element' for testing purposes.
-  html.HtmlElement _element;
+  html.HtmlElement? _element;
 
   html.HtmlElement get _domElement => _element ??= _createElement();
 
   /// Decodes the message coming from the 'flutter/accessibility' channel.
-  void handleMessage(StandardMessageCodec codec, ByteData data) {
+  void handleMessage(StandardMessageCodec codec, ByteData? data) {
     final Map<dynamic, dynamic> inputMap =
         codec.decodeMessage(data);
     final Map<dynamic, dynamic> dataMap = inputMap['data'];
-    final String message = dataMap['message'];
+    final String? message = dataMap['message'];
     if (message != null && message.isNotEmpty) {
       _initLiveRegion(message);
       _removeElementTimer = Timer(durationA11yMessageIsOnDom, () {
-        _element.remove();
+        _element!.remove();
       });
     }
   }
@@ -68,7 +68,7 @@ class AccessibilityAnnouncements {
   void _initLiveRegion(String message) {
     _domElement.setAttribute('aria-live', 'polite');
     _domElement.text = message;
-    html.document.body.append(_domElement);
+    html.document.body!.append(_domElement);
   }
 
   html.LabelElement _createElement() {
diff --git a/lib/web_ui/lib/src/engine/semantics/checkable.dart b/lib/web_ui/lib/src/engine/semantics/checkable.dart
index 8cf23b39d6770..c8d7f3ccfca8f 100644
--- a/lib/web_ui/lib/src/engine/semantics/checkable.dart
+++ b/lib/web_ui/lib/src/engine/semantics/checkable.dart
@@ -11,7 +11,7 @@
 //                framework. Currently the framework does not report the
 //                grouping of radio buttons.
 
-// @dart = 2.6
+
 part of engine;
 
 /// The specific type of checkable control.
@@ -28,6 +28,16 @@ enum _CheckableKind {
   toggle,
 }
 
+_CheckableKind _checkableKindFromSemanticsFlag(SemanticsObject semanticsObject) {
+  if (semanticsObject.hasFlag(ui.SemanticsFlag.isInMutuallyExclusiveGroup)) {
+    return _CheckableKind.radio;
+  } else if (semanticsObject.hasFlag(ui.SemanticsFlag.hasToggledState)) {
+    return _CheckableKind.toggle;
+  } else {
+    return _CheckableKind.checkbox;
+  }
+}
+
 /// Renders semantics objects that have checkable (on/off) states.
 ///
 /// Three objects which are implemented by this class are checkboxes, radio
@@ -37,18 +47,11 @@ enum _CheckableKind {
 /// [ui.SemanticsFlag.isInMutuallyExclusiveGroup], [ui.SemanticsFlag.isToggled],
 /// [ui.SemanticsFlag.hasToggledState]
 class Checkable extends RoleManager {
-  _CheckableKind _kind;
+  final _CheckableKind _kind;
 
   Checkable(SemanticsObject semanticsObject)
-      : super(Role.checkable, semanticsObject) {
-    if (semanticsObject.hasFlag(ui.SemanticsFlag.isInMutuallyExclusiveGroup)) {
-      _kind = _CheckableKind.radio;
-    } else if (semanticsObject.hasFlag(ui.SemanticsFlag.hasToggledState)) {
-      _kind = _CheckableKind.toggle;
-    } else {
-      _kind = _CheckableKind.checkbox;
-    }
-  }
+      : _kind = _checkableKindFromSemanticsFlag(semanticsObject),
+        super(Role.checkable, semanticsObject);
 
   @override
   void update() {
diff --git a/lib/web_ui/lib/src/engine/semantics/image.dart b/lib/web_ui/lib/src/engine/semantics/image.dart
index 1b97f46f0bb4c..52dd645de2071 100644
--- a/lib/web_ui/lib/src/engine/semantics/image.dart
+++ b/lib/web_ui/lib/src/engine/semantics/image.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Represents semantic objects that deliver information in a visual manner.
@@ -17,7 +17,7 @@ class ImageRoleManager extends RoleManager {
   /// The element with role="img" and aria-label could block access to all
   /// children elements, therefore create an auxiliary element and  describe the
   /// image in that if the semantic object have child nodes.
-  html.Element _auxiliaryImageElement;
+  html.Element? _auxiliaryImageElement;
 
   @override
   void update() {
@@ -30,18 +30,18 @@ class ImageRoleManager extends RoleManager {
         // other nodes, then VoiceOver behaves as expected with absolute
         // positioning and sizing.
         if (semanticsObject.hasChildren) {
-          _auxiliaryImageElement.style
+          _auxiliaryImageElement!.style
             ..position = 'absolute'
             ..top = '0'
             ..left = '0'
-            ..width = '${semanticsObject.rect.width}px'
-            ..height = '${semanticsObject.rect.height}px';
+            ..width = '${semanticsObject.rect!.width}px'
+            ..height = '${semanticsObject.rect!.height}px';
         }
-        _auxiliaryImageElement.style.fontSize = '6px';
-        semanticsObject.element.append(_auxiliaryImageElement);
+        _auxiliaryImageElement!.style.fontSize = '6px';
+        semanticsObject.element.append(_auxiliaryImageElement!);
       }
 
-      _auxiliaryImageElement.setAttribute('role', 'img');
+      _auxiliaryImageElement!.setAttribute('role', 'img');
       _setLabel(_auxiliaryImageElement);
     } else if (semanticsObject.isVisualOnly) {
       semanticsObject.setAriaRole('img', true);
@@ -53,15 +53,15 @@ class ImageRoleManager extends RoleManager {
     }
   }
 
-  void _setLabel(html.Element element) {
+  void _setLabel(html.Element? element) {
     if (semanticsObject.hasLabel) {
-      element.setAttribute('aria-label', semanticsObject.label);
+      element!.setAttribute('aria-label', semanticsObject.label!);
     }
   }
 
   void _cleanUpAuxiliaryElement() {
     if (_auxiliaryImageElement != null) {
-      _auxiliaryImageElement.remove();
+      _auxiliaryImageElement!.remove();
       _auxiliaryImageElement = null;
     }
   }
diff --git a/lib/web_ui/lib/src/engine/semantics/incrementable.dart b/lib/web_ui/lib/src/engine/semantics/incrementable.dart
index aec945a769b65..3a86c33494c35 100644
--- a/lib/web_ui/lib/src/engine/semantics/incrementable.dart
+++ b/lib/web_ui/lib/src/engine/semantics/incrementable.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
 /// Adds increment/decrement event handling to a semantics object.
@@ -30,7 +29,7 @@ class Incrementable extends RoleManager {
   /// Disables the input [_element] when the gesture mode switches to
   /// [GestureMode.pointerEvents], and enables it when the mode switches back to
   /// [GestureMode.browserGestures].
-  GestureModeCallback _gestureModeListener;
+  GestureModeCallback? _gestureModeListener;
 
   /// Whether we forwarded a semantics action to the framework and awaiting an
   /// update.
@@ -108,15 +107,15 @@ class Incrementable extends RoleManager {
     final String surrogateTextValue = '$_currentSurrogateValue';
     _element.value = surrogateTextValue;
     _element.setAttribute('aria-valuenow', surrogateTextValue);
-    _element.setAttribute('aria-valuetext', semanticsObject.value);
+    _element.setAttribute('aria-valuetext', semanticsObject.value!);
 
-    final bool canIncrease = semanticsObject.increasedValue.isNotEmpty;
+    final bool canIncrease = semanticsObject.increasedValue!.isNotEmpty;
     final String surrogateMaxTextValue =
         canIncrease ? '${_currentSurrogateValue + 1}' : surrogateTextValue;
     _element.max = surrogateMaxTextValue;
     _element.setAttribute('aria-valuemax', surrogateMaxTextValue);
 
-    final bool canDecrease = semanticsObject.decreasedValue.isNotEmpty;
+    final bool canDecrease = semanticsObject.decreasedValue!.isNotEmpty;
     final String surrogateMinTextValue =
         canDecrease ? '${_currentSurrogateValue - 1}' : surrogateTextValue;
     _element.min = surrogateMinTextValue;
diff --git a/lib/web_ui/lib/src/engine/semantics/label_and_value.dart b/lib/web_ui/lib/src/engine/semantics/label_and_value.dart
index de78e08ee6d4b..204146335c6a6 100644
--- a/lib/web_ui/lib/src/engine/semantics/label_and_value.dart
+++ b/lib/web_ui/lib/src/engine/semantics/label_and_value.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Renders [_label] and [_value] to the semantics DOM.
@@ -43,7 +43,7 @@ class LabelAndValue extends RoleManager {
   ///   its label is not reachable via accessibility focus. This happens, for
   ///   example in popup dialogs, such as the alert dialog. The text of the
   ///   alert is supplied as a label on the parent node.
-  html.Element _auxiliaryValueElement;
+  html.Element? _auxiliaryValueElement;
 
   @override
   void update() {
@@ -89,22 +89,22 @@ class LabelAndValue extends RoleManager {
       // nodes, then VoiceOver behaves as expected with absolute positioning and
       // sizing.
       if (semanticsObject.hasChildren) {
-        _auxiliaryValueElement.style
+        _auxiliaryValueElement!.style
           ..position = 'absolute'
           ..top = '0'
           ..left = '0'
-          ..width = '${semanticsObject.rect.width}px'
-          ..height = '${semanticsObject.rect.height}px';
+          ..width = '${semanticsObject.rect!.width}px'
+          ..height = '${semanticsObject.rect!.height}px';
       }
-      _auxiliaryValueElement.style.fontSize = '6px';
-      semanticsObject.element.append(_auxiliaryValueElement);
+      _auxiliaryValueElement!.style.fontSize = '6px';
+      semanticsObject.element.append(_auxiliaryValueElement!);
     }
-    _auxiliaryValueElement.text = combinedValue.toString();
+    _auxiliaryValueElement!.text = combinedValue.toString();
   }
 
   void _cleanUpDom() {
     if (_auxiliaryValueElement != null) {
-      _auxiliaryValueElement.remove();
+      _auxiliaryValueElement!.remove();
       _auxiliaryValueElement = null;
     }
     semanticsObject.element.attributes.remove('aria-label');
diff --git a/lib/web_ui/lib/src/engine/semantics/live_region.dart b/lib/web_ui/lib/src/engine/semantics/live_region.dart
index 8229d67c267b0..ccb6be47c1623 100644
--- a/lib/web_ui/lib/src/engine/semantics/live_region.dart
+++ b/lib/web_ui/lib/src/engine/semantics/live_region.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Manages semantics configurations that represent live regions.
diff --git a/lib/web_ui/lib/src/engine/semantics/scrollable.dart b/lib/web_ui/lib/src/engine/semantics/scrollable.dart
index 0c818314a26f1..ae99803291f2f 100644
--- a/lib/web_ui/lib/src/engine/semantics/scrollable.dart
+++ b/lib/web_ui/lib/src/engine/semantics/scrollable.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Implements vertical and horizontal scrolling functionality for semantics
@@ -27,13 +27,13 @@ class Scrollable extends RoleManager {
       : super(Role.scrollable, semanticsObject);
 
   /// Disables browser-driven scrolling in the presence of pointer events.
-  GestureModeCallback _gestureModeListener;
+  GestureModeCallback? _gestureModeListener;
 
   /// Listens to HTML "scroll" gestures detected by the browser.
   ///
   /// This gesture is converted to [ui.SemanticsAction.scrollUp] or
   /// [ui.SemanticsAction.scrollDown], depending on the direction.
-  html.EventListener _scrollListener;
+  html.EventListener? _scrollListener;
 
   /// The value of the "scrollTop" or "scrollLeft" property of this object's
   /// [element] that has zero offset relative to the [scrollPosition].
diff --git a/lib/web_ui/lib/src/engine/semantics/semantics.dart b/lib/web_ui/lib/src/engine/semantics/semantics.dart
index d1808ce2c223a..3d880a29d3de3 100644
--- a/lib/web_ui/lib/src/engine/semantics/semantics.dart
+++ b/lib/web_ui/lib/src/engine/semantics/semantics.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Set this flag to `true` to cause the engine to visualize the semantics tree
@@ -16,11 +16,11 @@ const bool _debugShowSemanticsNodes = false;
 /// This class provides private engine-side API that's not available in the
 /// `dart:ui` [ui.SemanticsUpdate].
 class SemanticsUpdate implements ui.SemanticsUpdate {
-  SemanticsUpdate({List<SemanticsNodeUpdate> nodeUpdates})
+  SemanticsUpdate({List<SemanticsNodeUpdate>? nodeUpdates})
       : _nodeUpdates = nodeUpdates;
 
   /// Updates for individual nodes.
-  final List<SemanticsNodeUpdate> _nodeUpdates;
+  final List<SemanticsNodeUpdate>? _nodeUpdates;
 
   @override
   void dispose() {
@@ -33,32 +33,32 @@ class SemanticsUpdate implements ui.SemanticsUpdate {
 /// Updates the properties of a particular semantics node.
 class SemanticsNodeUpdate {
   SemanticsNodeUpdate({
-    this.id,
-    this.flags,
-    this.actions,
-    this.maxValueLength,
-    this.currentValueLength,
-    this.textSelectionBase,
-    this.textSelectionExtent,
-    this.platformViewId,
-    this.scrollChildren,
-    this.scrollIndex,
-    this.scrollPosition,
-    this.scrollExtentMax,
-    this.scrollExtentMin,
-    this.rect,
-    this.label,
-    this.hint,
-    this.value,
-    this.increasedValue,
-    this.decreasedValue,
+    required this.id,
+    required this.flags,
+    required this.actions,
+    required this.maxValueLength,
+    required this.currentValueLength,
+    required this.textSelectionBase,
+    required this.textSelectionExtent,
+    required this.platformViewId,
+    required this.scrollChildren,
+    required this.scrollIndex,
+    required this.scrollPosition,
+    required this.scrollExtentMax,
+    required this.scrollExtentMin,
+    required this.rect,
+    required this.label,
+    required this.hint,
+    required this.value,
+    required this.increasedValue,
+    required this.decreasedValue,
     this.textDirection,
-    this.transform,
-    this.elevation,
-    this.thickness,
-    this.childrenInTraversalOrder,
-    this.childrenInHitTestOrder,
-    this.additionalActions,
+    required this.transform,
+    required this.elevation,
+    required this.thickness,
+    required this.childrenInTraversalOrder,
+    required this.childrenInHitTestOrder,
+    required this.additionalActions,
   });
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
@@ -119,7 +119,7 @@ class SemanticsNodeUpdate {
   final String decreasedValue;
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  final ui.TextDirection textDirection;
+  final ui.TextDirection? textDirection;
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
   final Float32List transform;
@@ -199,7 +199,7 @@ abstract class RoleManager {
   ///
   /// A single role object manages exactly one [SemanticsObject].
   RoleManager(this.role, this.semanticsObject)
-      : assert(semanticsObject != null);
+      : assert(semanticsObject != null); // ignore: unnecessary_null_comparison
 
   /// Role identifier.
   final Role role;
@@ -258,8 +258,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  int get flags => _flags;
-  int _flags;
+  int? get flags => _flags;
+  int? _flags;
 
   /// Whether the [flags] field has been updated but has not been applied to the
   /// DOM yet.
@@ -270,8 +270,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  int get actions => _actions;
-  int _actions;
+  int? get actions => _actions;
+  int? _actions;
 
   static const int _actionsIndex = 1 << 1;
 
@@ -283,8 +283,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  int get textSelectionBase => _textSelectionBase;
-  int _textSelectionBase;
+  int? get textSelectionBase => _textSelectionBase;
+  int? _textSelectionBase;
 
   static const int _textSelectionBaseIndex = 1 << 2;
 
@@ -296,8 +296,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  int get textSelectionExtent => _textSelectionExtent;
-  int _textSelectionExtent;
+  int? get textSelectionExtent => _textSelectionExtent;
+  int? _textSelectionExtent;
 
   static const int _textSelectionExtentIndex = 1 << 3;
 
@@ -309,8 +309,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  int get scrollChildren => _scrollChildren;
-  int _scrollChildren;
+  int? get scrollChildren => _scrollChildren;
+  int? _scrollChildren;
 
   static const int _scrollChildrenIndex = 1 << 4;
 
@@ -322,8 +322,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  int get scrollIndex => _scrollIndex;
-  int _scrollIndex;
+  int? get scrollIndex => _scrollIndex;
+  int? _scrollIndex;
 
   static const int _scrollIndexIndex = 1 << 5;
 
@@ -335,8 +335,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  double get scrollPosition => _scrollPosition;
-  double _scrollPosition;
+  double? get scrollPosition => _scrollPosition;
+  double? _scrollPosition;
 
   static const int _scrollPositionIndex = 1 << 6;
 
@@ -348,8 +348,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  double get scrollExtentMax => _scrollExtentMax;
-  double _scrollExtentMax;
+  double? get scrollExtentMax => _scrollExtentMax;
+  double? _scrollExtentMax;
 
   static const int _scrollExtentMaxIndex = 1 << 7;
 
@@ -361,8 +361,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  double get scrollExtentMin => _scrollExtentMin;
-  double _scrollExtentMin;
+  double? get scrollExtentMin => _scrollExtentMin;
+  double? _scrollExtentMin;
 
   static const int _scrollExtentMinIndex = 1 << 8;
 
@@ -374,8 +374,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  ui.Rect get rect => _rect;
-  ui.Rect _rect;
+  ui.Rect? get rect => _rect;
+  ui.Rect? _rect;
 
   static const int _rectIndex = 1 << 9;
 
@@ -387,11 +387,11 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  String get label => _label;
-  String _label;
+  String? get label => _label;
+  String? _label;
 
   /// Whether this object contains a non-empty label.
-  bool get hasLabel => _label != null && _label.isNotEmpty;
+  bool get hasLabel => _label != null && _label!.isNotEmpty;
 
   static const int _labelIndex = 1 << 10;
 
@@ -403,8 +403,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  String get hint => _hint;
-  String _hint;
+  String? get hint => _hint;
+  String? _hint;
 
   static const int _hintIndex = 1 << 11;
 
@@ -416,11 +416,11 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  String get value => _value;
-  String _value;
+  String? get value => _value;
+  String? _value;
 
   /// Whether this object contains a non-empty value.
-  bool get hasValue => _value != null && _value.isNotEmpty;
+  bool get hasValue => _value != null && _value!.isNotEmpty;
 
   static const int _valueIndex = 1 << 12;
 
@@ -432,8 +432,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  String get increasedValue => _increasedValue;
-  String _increasedValue;
+  String? get increasedValue => _increasedValue;
+  String? _increasedValue;
 
   static const int _increasedValueIndex = 1 << 13;
 
@@ -445,8 +445,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  String get decreasedValue => _decreasedValue;
-  String _decreasedValue;
+  String? get decreasedValue => _decreasedValue;
+  String? _decreasedValue;
 
   static const int _decreasedValueIndex = 1 << 14;
 
@@ -458,8 +458,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  ui.TextDirection get textDirection => _textDirection;
-  ui.TextDirection _textDirection;
+  ui.TextDirection? get textDirection => _textDirection;
+  ui.TextDirection? _textDirection;
 
   static const int _textDirectionIndex = 1 << 15;
 
@@ -471,8 +471,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  Float32List get transform => _transform;
-  Float32List _transform;
+  Float32List? get transform => _transform;
+  Float32List? _transform;
 
   static const int _transformIndex = 1 << 16;
 
@@ -484,8 +484,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  Int32List get childrenInTraversalOrder => _childrenInTraversalOrder;
-  Int32List _childrenInTraversalOrder;
+  Int32List? get childrenInTraversalOrder => _childrenInTraversalOrder;
+  Int32List? _childrenInTraversalOrder;
 
   static const int _childrenInTraversalOrderIndex = 1 << 19;
 
@@ -498,8 +498,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  Int32List get childrenInHitTestOrder => _childrenInHitTestOrder;
-  Int32List _childrenInHitTestOrder;
+  Int32List? get childrenInHitTestOrder => _childrenInHitTestOrder;
+  Int32List? _childrenInHitTestOrder;
 
   static const int _childrenInHitTestOrderIndex = 1 << 20;
 
@@ -512,8 +512,8 @@ class SemanticsObject {
   }
 
   /// See [ui.SemanticsUpdateBuilder.updateNode].
-  Int32List get additionalActions => _additionalActions;
-  Int32List _additionalActions;
+  Int32List? get additionalActions => _additionalActions;
+  Int32List? _additionalActions;
 
   static const int _additionalActionsIndex = 1 << 21;
 
@@ -552,11 +552,11 @@ class SemanticsObject {
   /// is not created. This is necessary for "aria-label" to function correctly.
   /// The browser will ignore the [label] of HTML element that contain child
   /// elements.
-  html.Element getOrCreateChildContainer() {
+  html.Element? getOrCreateChildContainer() {
     if (_childContainerElement == null) {
       _childContainerElement = html.Element.tag('flt-semantics-container');
-      _childContainerElement.style.position = 'absolute';
-      element.append(_childContainerElement);
+      _childContainerElement!.style.position = 'absolute';
+      element.append(_childContainerElement!);
     }
     return _childContainerElement;
   }
@@ -566,16 +566,16 @@ class SemanticsObject {
   ///
   /// This element is used to correct for [_rect] offsets. It is only non-`null`
   /// when there are non-zero children (i.e. when [hasChildren] is `true`).
-  html.Element _childContainerElement;
+  html.Element? _childContainerElement;
 
   /// The parent of this semantics object.
-  SemanticsObject _parent;
+  SemanticsObject? _parent;
 
   /// Whether this node currently has a given [SemanticsFlag].
-  bool hasFlag(ui.SemanticsFlag flag) => _flags & flag.index != 0;
+  bool hasFlag(ui.SemanticsFlag flag) => _flags! & flag.index != 0;
 
   /// Whether [actions] contains the given action.
-  bool hasAction(ui.SemanticsAction action) => (_actions & action.index) != 0;
+  bool hasAction(ui.SemanticsAction action) => (_actions! & action.index) != 0;
 
   /// Whether this object represents a vertically scrollable area.
   bool get isVerticalScrollContainer =>
@@ -589,7 +589,7 @@ class SemanticsObject {
 
   /// Whether this object has a non-empty list of children.
   bool get hasChildren =>
-      _childrenInTraversalOrder != null && _childrenInTraversalOrder.isNotEmpty;
+      _childrenInTraversalOrder != null && _childrenInTraversalOrder!.isNotEmpty;
 
   /// Whether this object represents an editable text field.
   bool get isTextField => hasFlag(ui.SemanticsFlag.isTextField);
@@ -612,7 +612,7 @@ class SemanticsObject {
   void updateWith(SemanticsNodeUpdate update) {
     // Update all field values and their corresponding dirty flags before
     // applying the updates to the DOM.
-    assert(update.flags != null);
+    assert(update.flags != null); // ignore: unnecessary_null_comparison
     if (_flags != update.flags) {
       _flags = update.flags;
       _markFlagsDirty();
@@ -753,7 +753,7 @@ class SemanticsObject {
   /// spec:
   ///
   /// > A map literal is ordered: iterating over the keys and/or values of the maps always happens in the order the keys appeared in the source code.
-  final Map<Role, RoleManager> _roleManagers = <Role, RoleManager>{};
+  final Map<Role, RoleManager?> _roleManagers = <Role, RoleManager?>{};
 
   /// Detects the roles that this semantics object corresponds to and manages
   /// the lifecycles of [SemanticsObjectRole] objects.
@@ -776,10 +776,10 @@ class SemanticsObject {
   }
 
   void _updateRole(Role role, bool enabled) {
-    RoleManager manager = _roleManagers[role];
+    RoleManager? manager = _roleManagers[role];
     if (enabled) {
       if (manager == null) {
-        manager = _roleFactories[role](this);
+        manager = _roleFactories[role]!(this);
         _roleManagers[role] = manager;
       }
       manager.update();
@@ -820,15 +820,16 @@ class SemanticsObject {
   /// [hasChildren], of [getOrCreateChildContainer].
   void recomputePositionAndSize() {
     element.style
-      ..width = '${_rect.width}px'
-      ..height = '${_rect.height}px';
+      ..width = '${_rect!.width}px'
+      ..height = '${_rect!.height}px';
 
-    final html.Element containerElement =
+    final html.Element? containerElement =
         hasChildren ? getOrCreateChildContainer() : null;
 
-    final bool hasZeroRectOffset = _rect.top == 0.0 && _rect.left == 0.0;
+    final bool hasZeroRectOffset = _rect!.top == 0.0 && _rect!.left == 0.0;
+    final Float32List? transform = _transform;
     final bool hasIdentityTransform =
-        _transform == null || isIdentityFloat32ListTransform(_transform);
+        transform == null || isIdentityFloat32ListTransform(transform);
 
     if (hasZeroRectOffset &&
         hasIdentityTransform &&
@@ -845,22 +846,22 @@ class SemanticsObject {
       return;
     }
 
-    Matrix4 effectiveTransform;
+    late Matrix4 effectiveTransform;
     bool effectiveTransformIsIdentity = true;
     if (!hasZeroRectOffset) {
-      if (_transform == null) {
-        final double left = _rect.left;
-        final double top = _rect.top;
+      if (transform == null) {
+        final double left = _rect!.left;
+        final double top = _rect!.top;
         effectiveTransform = Matrix4.translationValues(left, top, 0.0);
         effectiveTransformIsIdentity = left == 0.0 && top == 0.0;
       } else {
         // Clone to avoid mutating _transform.
-        effectiveTransform = Matrix4.fromFloat32List(_transform).clone()
-          ..translate(_rect.left, _rect.top, 0.0);
+        effectiveTransform = Matrix4.fromFloat32List(transform).clone()
+          ..translate(_rect!.left, _rect!.top, 0.0);
         effectiveTransformIsIdentity = effectiveTransform.isIdentity();
       }
     } else if (!hasIdentityTransform) {
-      effectiveTransform = Matrix4.fromFloat32List(_transform);
+      effectiveTransform = Matrix4.fromFloat32List(transform!);
       effectiveTransformIsIdentity = false;
     }
 
@@ -878,8 +879,8 @@ class SemanticsObject {
       if (!hasZeroRectOffset ||
           verticalContainerAdjustment != 0.0 ||
           horizontalContainerAdjustment != 0.0) {
-        final double translateX = -_rect.left + horizontalContainerAdjustment;
-        final double translateY = -_rect.top + verticalContainerAdjustment;
+        final double translateX = -_rect!.left + horizontalContainerAdjustment;
+        final double translateY = -_rect!.top + verticalContainerAdjustment;
         containerElement.style
           ..transformOrigin = '0 0 0'
           ..transform = 'translate(${translateX}px, ${translateY}px)';
@@ -891,7 +892,7 @@ class SemanticsObject {
     }
   }
 
-  Int32List _previousChildrenInTraversalOrder;
+  Int32List? _previousChildrenInTraversalOrder;
 
   /// Updates the traversal child list of [object] from the given [update].
   ///
@@ -901,9 +902,9 @@ class SemanticsObject {
   void _updateChildrenInTraversalOrder() {
     // Remove all children case.
     if (_childrenInTraversalOrder == null ||
-        _childrenInTraversalOrder.isEmpty) {
+        _childrenInTraversalOrder!.isEmpty) {
       if (_previousChildrenInTraversalOrder == null ||
-          _previousChildrenInTraversalOrder.isEmpty) {
+          _previousChildrenInTraversalOrder!.isEmpty) {
         // We must not have created a container element when child list is empty.
         assert(_childContainerElement == null);
         _previousChildrenInTraversalOrder = _childrenInTraversalOrder;
@@ -914,26 +915,26 @@ class SemanticsObject {
       assert(_childContainerElement != null);
 
       // Remove all children from this semantics object.
-      final int len = _previousChildrenInTraversalOrder.length;
+      final int len = _previousChildrenInTraversalOrder!.length;
       for (int i = 0; i < len; i++) {
-        owner._detachObject(_previousChildrenInTraversalOrder[i]);
+        owner._detachObject(_previousChildrenInTraversalOrder![i]);
       }
       _previousChildrenInTraversalOrder = null;
-      _childContainerElement.remove();
+      _childContainerElement!.remove();
       _childContainerElement = null;
       _previousChildrenInTraversalOrder = _childrenInTraversalOrder;
       return;
     }
 
-    final html.Element containerElement = getOrCreateChildContainer();
+    final html.Element? containerElement = getOrCreateChildContainer();
 
     // Empty case.
     if (_previousChildrenInTraversalOrder == null ||
-        _previousChildrenInTraversalOrder.isEmpty) {
+        _previousChildrenInTraversalOrder!.isEmpty) {
       _previousChildrenInTraversalOrder = _childrenInTraversalOrder;
-      for (int id in _previousChildrenInTraversalOrder) {
+      for (int id in _previousChildrenInTraversalOrder!) {
         final SemanticsObject child = owner.getOrCreateObject(id);
-        containerElement.append(child.element);
+        containerElement!.append(child.element);
         owner._attachObject(parent: this, child: child);
       }
       _previousChildrenInTraversalOrder = _childrenInTraversalOrder;
@@ -954,34 +955,34 @@ class SemanticsObject {
 
     // The smallest of the two child list lengths.
     final int minLength = math.min(
-      _previousChildrenInTraversalOrder.length,
-      _childrenInTraversalOrder.length,
+      _previousChildrenInTraversalOrder!.length,
+      _childrenInTraversalOrder!.length,
     );
 
     // Scan forward until first discrepancy.
     while (newIndex < minLength &&
-        _previousChildrenInTraversalOrder[newIndex] ==
-            _childrenInTraversalOrder[newIndex]) {
+        _previousChildrenInTraversalOrder![newIndex] ==
+            _childrenInTraversalOrder![newIndex]) {
       intersectionIndicesNew.add(newIndex);
       intersectionIndicesOld.add(newIndex);
       newIndex += 1;
     }
 
     // If child lists are identical, do nothing.
-    if (_previousChildrenInTraversalOrder.length ==
-            _childrenInTraversalOrder.length &&
-        newIndex == _childrenInTraversalOrder.length) {
+    if (_previousChildrenInTraversalOrder!.length ==
+            _childrenInTraversalOrder!.length &&
+        newIndex == _childrenInTraversalOrder!.length) {
       return;
     }
 
     // If child lists are not identical, continue computing the intersection
     // between the two lists.
-    while (newIndex < _childrenInTraversalOrder.length) {
+    while (newIndex < _childrenInTraversalOrder!.length) {
       for (int oldIndex = 0;
-          oldIndex < _previousChildrenInTraversalOrder.length;
+          oldIndex < _previousChildrenInTraversalOrder!.length;
           oldIndex += 1) {
-        if (_previousChildrenInTraversalOrder[oldIndex] ==
-            _childrenInTraversalOrder[newIndex]) {
+        if (_previousChildrenInTraversalOrder![oldIndex] ==
+            _childrenInTraversalOrder![newIndex]) {
           intersectionIndicesNew.add(newIndex);
           intersectionIndicesOld.add(oldIndex);
           break;
@@ -992,32 +993,32 @@ class SemanticsObject {
 
     // The longest sub-sequence in the old list maximizes the number of children
     // that do not need to be moved.
-    final List<int> longestSequence =
+    final List<int?> longestSequence =
         longestIncreasingSubsequence(intersectionIndicesOld);
     final List<int> stationaryIds = <int>[];
     for (int i = 0; i < longestSequence.length; i += 1) {
-      stationaryIds.add(_previousChildrenInTraversalOrder[
-          intersectionIndicesOld[longestSequence[i]]]);
+      stationaryIds.add(_previousChildrenInTraversalOrder![
+          intersectionIndicesOld[longestSequence[i]!]]);
     }
 
     // Remove children that are no longer in the list.
-    for (int i = 0; i < _previousChildrenInTraversalOrder.length; i++) {
+    for (int i = 0; i < _previousChildrenInTraversalOrder!.length; i++) {
       if (!intersectionIndicesOld.contains(i)) {
         // Child not in the intersection. Must be removed.
-        final int childId = _previousChildrenInTraversalOrder[i];
+        final int childId = _previousChildrenInTraversalOrder![i];
         owner._detachObject(childId);
       }
     }
 
-    html.Element refNode;
-    for (int i = _childrenInTraversalOrder.length - 1; i >= 0; i -= 1) {
-      final int childId = _childrenInTraversalOrder[i];
+    html.Element? refNode;
+    for (int i = _childrenInTraversalOrder!.length - 1; i >= 0; i -= 1) {
+      final int childId = _childrenInTraversalOrder![i];
       final SemanticsObject child = owner.getOrCreateObject(childId);
       if (!stationaryIds.contains(childId)) {
         if (refNode == null) {
-          containerElement.append(child.element);
+          containerElement!.append(child.element);
         } else {
-          containerElement.insertBefore(child.element, refNode);
+          containerElement!.insertBefore(child.element, refNode);
         }
         owner._attachObject(parent: this, child: child);
       } else {
@@ -1033,8 +1034,8 @@ class SemanticsObject {
   String toString() {
     if (assertionsEnabled) {
       final String children = _childrenInTraversalOrder != null &&
-              _childrenInTraversalOrder.isNotEmpty
-          ? '[${_childrenInTraversalOrder.join(', ')}]'
+              _childrenInTraversalOrder!.isNotEmpty
+          ? '[${_childrenInTraversalOrder!.join(', ')}]'
           : '<empty>';
       return '$runtimeType(#$id, children: $children)';
     } else {
@@ -1085,7 +1086,7 @@ class EngineSemanticsOwner {
     return _instance ??= EngineSemanticsOwner._();
   }
 
-  static EngineSemanticsOwner _instance;
+  static EngineSemanticsOwner? _instance;
 
   /// Disables semantics and uninitializes the singleton [instance].
   ///
@@ -1096,24 +1097,24 @@ class EngineSemanticsOwner {
     if (_instance == null) {
       return;
     }
-    _instance.semanticsEnabled = false;
+    _instance!.semanticsEnabled = false;
     _instance = null;
   }
 
-  final Map<int, SemanticsObject> _semanticsTree = <int, SemanticsObject>{};
+  final Map<int?, SemanticsObject?> _semanticsTree = <int?, SemanticsObject?>{};
 
   /// Map [SemanticsObject.id] to parent [SemanticsObject] it was attached to
   /// this frame.
-  Map<int, SemanticsObject> _attachments = <int, SemanticsObject>{};
+  Map<int?, SemanticsObject> _attachments = <int?, SemanticsObject>{};
 
   /// Declares that the [child] must be attached to the [parent].
   ///
   /// Attachments take precendence over detachments (see [_detachObject]). This
   /// allows the same node to be detached from one parent in the tree and
   /// reattached to another parent.
-  void _attachObject({SemanticsObject parent, SemanticsObject child}) {
-    assert(child != null);
-    assert(parent != null);
+  void _attachObject({required SemanticsObject parent, required SemanticsObject child}) {
+    assert(child != null); // ignore: unnecessary_null_comparison
+    assert(parent != null); // ignore: unnecessary_null_comparison
     child._parent = parent;
     _attachments[child.id] = parent;
   }
@@ -1122,16 +1123,16 @@ class EngineSemanticsOwner {
   ///
   /// The objects in this list will be detached permanently unless they are
   /// reattached via the [_attachObject] method.
-  List<SemanticsObject> _detachments = <SemanticsObject>[];
+  List<SemanticsObject?> _detachments = <SemanticsObject?>[];
 
   /// Declares that the [SemanticsObject] with the given [id] was detached from
   /// its current parent object.
   ///
   /// The object will be detached permanently unless it is reattached via the
   /// [_attachObject] method.
-  void _detachObject(int id) {
+  void _detachObject(int? id) {
     assert(_semanticsTree.containsKey(id));
-    final SemanticsObject object = _semanticsTree[id];
+    final SemanticsObject? object = _semanticsTree[id];
     _detachments.add(object);
   }
 
@@ -1151,8 +1152,8 @@ class EngineSemanticsOwner {
   /// the one-time callbacks scheduled via the [addOneTimePostUpdateCallback]
   /// method.
   void _finalizeTree() {
-    for (SemanticsObject object in _detachments) {
-      final SemanticsObject parent = _attachments[object.id];
+    for (SemanticsObject? object in _detachments) {
+      final SemanticsObject? parent = _attachments[object!.id];
       if (parent == null) {
         // Was not reparented and is removed permanently from the tree.
         _semanticsTree.remove(object.id);
@@ -1163,8 +1164,8 @@ class EngineSemanticsOwner {
         assert(object.element.parent == parent._childContainerElement);
       }
     }
-    _detachments = <SemanticsObject>[];
-    _attachments = <int, SemanticsObject>{};
+    _detachments = <SemanticsObject?>[];
+    _attachments = <int?, SemanticsObject>{};
 
     if (_oneTimePostUpdateCallbacks.isNotEmpty) {
       for (ui.VoidCallback callback in _oneTimePostUpdateCallbacks) {
@@ -1177,8 +1178,8 @@ class EngineSemanticsOwner {
   /// Returns the entire semantics tree for testing.
   ///
   /// Works only in debug mode.
-  Map<int, SemanticsObject> get debugSemanticsTree {
-    Map<int, SemanticsObject> result;
+  Map<int?, SemanticsObject?>? get debugSemanticsTree {
+    Map<int?, SemanticsObject?>? result;
     assert(() {
       result = _semanticsTree;
       return true;
@@ -1187,7 +1188,7 @@ class EngineSemanticsOwner {
   }
 
   /// The top-level DOM element of the semantics DOM element tree.
-  html.Element _rootSemanticsElement;
+  html.Element? _rootSemanticsElement;
 
   TimestampFunction _now = () => DateTime.now();
 
@@ -1225,7 +1226,7 @@ class EngineSemanticsOwner {
         _gestureMode = GestureMode.pointerEvents;
         _notifyGestureModeListeners();
       }
-      final List<int> keys = _semanticsTree.keys.toList();
+      final List<int?> keys = _semanticsTree.keys.toList();
       final int len = keys.length;
       for (int i = 0; i < len; i++) {
         _detachObject(keys[i]);
@@ -1247,7 +1248,7 @@ class EngineSemanticsOwner {
   /// The default mode is [AccessibilityMode.unknown].
   AccessibilityMode get mode => _mode;
   set mode(AccessibilityMode value) {
-    assert(value != null);
+    assert(value != null); // ignore: unnecessary_null_comparison
     _mode = value;
   }
 
@@ -1262,12 +1263,12 @@ class EngineSemanticsOwner {
   GestureMode get gestureMode => _gestureMode;
   GestureMode _gestureMode = GestureMode.browserGestures;
 
-  AlarmClock _gestureModeClock;
+  AlarmClock? _gestureModeClock;
 
-  AlarmClock _getGestureModeClock() {
+  AlarmClock? _getGestureModeClock() {
     if (_gestureModeClock == null) {
       _gestureModeClock = AlarmClock(_now);
-      _gestureModeClock.callback = () {
+      _gestureModeClock!.callback = () {
         if (_gestureMode == GestureMode.browserGestures) {
           return;
         }
@@ -1286,7 +1287,7 @@ class EngineSemanticsOwner {
   /// detected by the browser. Flutter-detected gestures have higher precedence.
   void _temporarilyDisableBrowserGestureMode() {
     const Duration _kDebounceThreshold = Duration(milliseconds: 500);
-    _getGestureModeClock().datetime = _now().add(_kDebounceThreshold);
+    _getGestureModeClock()!.datetime = _now().add(_kDebounceThreshold);
     if (_gestureMode != GestureMode.pointerEvents) {
       _gestureMode = GestureMode.pointerEvents;
       _notifyGestureModeListeners();
@@ -1355,14 +1356,14 @@ class EngineSemanticsOwner {
   /// Callbacks are called synchronously. HTML DOM updates made in a callback
   /// take effect in the current animation frame and/or the current message loop
   /// event.
-  List<GestureModeCallback> _gestureModeListeners = <GestureModeCallback>[];
+  List<GestureModeCallback?> _gestureModeListeners = <GestureModeCallback?>[];
 
   /// Calls the [callback] every time the current [GestureMode] changes.
   ///
   /// The callback is called synchronously. HTML DOM updates made in the
   /// callback take effect in the current animation frame and/or the current
   /// message loop event.
-  void addGestureModeListener(GestureModeCallback callback) {
+  void addGestureModeListener(GestureModeCallback? callback) {
     _gestureModeListeners.add(callback);
   }
 
@@ -1370,14 +1371,14 @@ class EngineSemanticsOwner {
   ///
   /// The passed [callback] must be the exact same object as the one passed to
   /// [addGestureModeListener].
-  void removeGestureModeListener(GestureModeCallback callback) {
+  void removeGestureModeListener(GestureModeCallback? callback) {
     assert(_gestureModeListeners.contains(callback));
     _gestureModeListeners.remove(callback);
   }
 
   void _notifyGestureModeListeners() {
     for (int i = 0; i < _gestureModeListeners.length; i++) {
-      _gestureModeListeners[i](_gestureMode);
+      _gestureModeListeners[i]!(_gestureMode);
     }
   }
 
@@ -1413,7 +1414,7 @@ class EngineSemanticsOwner {
   /// Looks up a [SemanticsObject] in the semantics tree by ID, or creates a new
   /// instance if it does not exist.
   SemanticsObject getOrCreateObject(int id) {
-    SemanticsObject object = _semanticsTree[id];
+    SemanticsObject? object = _semanticsTree[id];
     if (object == null) {
       object = SemanticsObject(id, this);
       _semanticsTree[id] = object;
@@ -1422,19 +1423,19 @@ class EngineSemanticsOwner {
   }
 
   /// Updates the semantics tree from data in the [uiUpdate].
-  void updateSemantics(ui.SemanticsUpdate/*!*/ uiUpdate) {
+  void updateSemantics(ui.SemanticsUpdate uiUpdate) {
     if (!_semanticsEnabled) {
       return;
     }
 
-    final SemanticsUpdate update = uiUpdate;
-    for (SemanticsNodeUpdate nodeUpdate in update._nodeUpdates) {
+    final SemanticsUpdate update = uiUpdate as SemanticsUpdate;
+    for (SemanticsNodeUpdate nodeUpdate in update._nodeUpdates!) {
       final SemanticsObject object = getOrCreateObject(nodeUpdate.id);
       object.updateWith(nodeUpdate);
     }
 
     if (_rootSemanticsElement == null) {
-      final SemanticsObject root = _semanticsTree[0];
+      final SemanticsObject root = _semanticsTree[0]!;
       _rootSemanticsElement = root.element;
       // We render semantics inside the glasspane for proper focus and event
       // handling. If semantics is behind the glasspane, the phone will disable
@@ -1447,8 +1448,8 @@ class EngineSemanticsOwner {
       // pointer events through to the semantics tree. However, for platform
       // views, the pointer events will not pass through, and will be handled
       // by the platform view.
-      domRenderer.glassPaneElement
-          .insertBefore(_rootSemanticsElement, domRenderer.sceneHostElement);
+      domRenderer.glassPaneElement!
+          .insertBefore(_rootSemanticsElement!, domRenderer.sceneHostElement);
     }
 
     _finalizeTree();
@@ -1456,13 +1457,13 @@ class EngineSemanticsOwner {
     assert(_semanticsTree.containsKey(0)); // must contain root node
     assert(() {
       // Validate tree
-      _semanticsTree.forEach((int id, SemanticsObject object) {
-        assert(id == object.id);
+      _semanticsTree.forEach((int? id, SemanticsObject? object) {
+        assert(id == object!.id);
         // Ensure child ID list is consistent with the parent-child
         // relationship of the semantics tree.
-        if (object._childrenInTraversalOrder != null) {
-          for (int childId in object._childrenInTraversalOrder) {
-            final SemanticsObject child = _semanticsTree[childId];
+        if (object!._childrenInTraversalOrder != null) {
+          for (int childId in object._childrenInTraversalOrder!) {
+            final SemanticsObject? child = _semanticsTree[childId];
             if (child == null) {
               throw AssertionError('Child #$childId is missing in the tree.');
             }
@@ -1474,18 +1475,16 @@ class EngineSemanticsOwner {
             if (!identical(child._parent, object)) {
               throw AssertionError(
                   'Parent #${object.id} has child #$childId. However, the '
-                  'child is attached to #${child._parent.id}.');
+                  'child is attached to #${child._parent!.id}.');
             }
           }
         }
       });
 
       // Validate that all updates were applied
-      for (SemanticsNodeUpdate update in update._nodeUpdates) {
+      for (SemanticsNodeUpdate update in update._nodeUpdates!) {
         // Node was added to the tree.
         assert(_semanticsTree.containsKey(update.id));
-        // We created a DOM element for it.
-        assert(_semanticsTree[update.id].element != null);
       }
 
       return true;
@@ -1535,7 +1534,7 @@ List<int> longestIncreasingSubsequence(List<int> list) {
     }
   }
   // Reconstruct the longest subsequence
-  final List<int> seq = List<int>(longest);
+  final List<int> seq = List<int>.filled(longest, 0);
   int k = mins[longest];
   for (int i = longest - 1; i >= 0; i--) {
     seq[i] = k;
diff --git a/lib/web_ui/lib/src/engine/semantics/semantics_helper.dart b/lib/web_ui/lib/src/engine/semantics/semantics_helper.dart
index cc77d76498ba1..b9eff3e892ce6 100644
--- a/lib/web_ui/lib/src/engine/semantics/semantics_helper.dart
+++ b/lib/web_ui/lib/src/engine/semantics/semantics_helper.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// The maximum [semanticsActivationAttempts] before we give up waiting for
@@ -107,10 +107,10 @@ class DesktopSemanticsEnabler extends SemanticsEnabler {
   /// request to activate semantics, and a second time by Flutter's gesture
   /// recognizers.
   @visibleForTesting
-  Timer semanticsActivationTimer;
+  Timer? semanticsActivationTimer;
 
   /// A temporary placeholder used to capture a request to activate semantics.
-  html.Element _semanticsPlaceholder;
+  html.Element? _semanticsPlaceholder;
 
   /// The number of events we processed that could potentially activate
   /// semantics.
@@ -128,7 +128,7 @@ class DesktopSemanticsEnabler extends SemanticsEnabler {
   @override
   bool tryEnableSemantics(html.Event event) {
     if (_schedulePlaceholderRemoval) {
-      _semanticsPlaceholder.remove();
+      _semanticsPlaceholder!.remove();
       _semanticsPlaceholder = null;
       semanticsActivationTimer = null;
       return true;
@@ -190,11 +190,11 @@ class DesktopSemanticsEnabler extends SemanticsEnabler {
 
   @override
   html.Element prepareAccesibilityPlaceholder() {
-    _semanticsPlaceholder = html.Element.tag('flt-semantics-placeholder');
+    final html.Element placeholder = _semanticsPlaceholder = html.Element.tag('flt-semantics-placeholder');
 
     // Only listen to "click" because other kinds of events are reported via
     // PointerBinding.
-    _semanticsPlaceholder.addEventListener('click', (html.Event event) {
+    placeholder.addEventListener('click', (html.Event event) {
       tryEnableSemantics(event);
     }, true);
 
@@ -203,18 +203,18 @@ class DesktopSemanticsEnabler extends SemanticsEnabler {
     // page receives focus. 'tab-index' makes sure the button is the first
     // target of tab. 'aria-label' is used to define the placeholder message
     // to the assistive technology user.
-    _semanticsPlaceholder
+    placeholder
       ..setAttribute('role', 'button')
       ..setAttribute('aria-live', 'true')
       ..setAttribute('tabindex', '0')
       ..setAttribute('aria-label', placeholderMessage);
-    _semanticsPlaceholder.style
+    placeholder.style
       ..position = 'absolute'
       ..left = '-1px'
       ..top = '-1px'
       ..width = '1px'
       ..height = '1px';
-    return _semanticsPlaceholder;
+    return placeholder;
   }
 }
 
@@ -230,10 +230,10 @@ class MobileSemanticsEnabler extends SemanticsEnabler {
   /// a request to activate semantics, and a second time by Flutter's gesture
   /// recognizers.
   @visibleForTesting
-  Timer semanticsActivationTimer;
+  Timer? semanticsActivationTimer;
 
   /// A temporary placeholder used to capture a request to activate semantics.
-  html.Element _semanticsPlaceholder;
+  html.Element? _semanticsPlaceholder;
 
   /// The number of events we processed that could potentially activate
   /// semantics.
@@ -259,7 +259,7 @@ class MobileSemanticsEnabler extends SemanticsEnabler {
       final bool removeNow =
           (browserEngine != BrowserEngine.webkit || event.type == 'touchend');
       if (removeNow) {
-        _semanticsPlaceholder.remove();
+        _semanticsPlaceholder!.remove();
         _semanticsPlaceholder = null;
         semanticsActivationTimer = null;
       }
@@ -325,12 +325,12 @@ class MobileSemanticsEnabler extends SemanticsEnabler {
 
       switch (event.type) {
         case 'click':
-          final html.MouseEvent click = event;
+          final html.MouseEvent click = event as html.MouseEvent;
           activationPoint = click.offset;
           break;
         case 'touchstart':
         case 'touchend':
-          final html.TouchEvent touch = event;
+          final html.TouchEvent touch = event as html.TouchEvent;
           activationPoint = touch.changedTouches.first.client;
           break;
         default:
@@ -338,16 +338,14 @@ class MobileSemanticsEnabler extends SemanticsEnabler {
           return true;
       }
 
-      assert(activationPoint != null);
-
       final html.Rectangle<num> activatingElementRect =
-          domRenderer.glassPaneElement.getBoundingClientRect();
+          domRenderer.glassPaneElement!.getBoundingClientRect();
       final double midX = activatingElementRect.left +
-          (activatingElementRect.right - activatingElementRect.left) / 2;
+          (activatingElementRect.right - activatingElementRect.left) / 2 as double;
       final double midY = activatingElementRect.top +
-          (activatingElementRect.bottom - activatingElementRect.top) / 2;
-      final double deltaX = activationPoint.x - midX;
-      final double deltaY = activationPoint.y - midY;
+          (activatingElementRect.bottom - activatingElementRect.top) / 2 as double;
+      final double deltaX = activationPoint.x - midX as double;
+      final double deltaY = activationPoint.y - midY as double;
       final double deltaSquared = deltaX * deltaX + deltaY * deltaY;
       if (deltaSquared < 1.0) {
         safariEnableConditionPassed = true;
@@ -369,24 +367,24 @@ class MobileSemanticsEnabler extends SemanticsEnabler {
 
   @override
   html.Element prepareAccesibilityPlaceholder() {
-    _semanticsPlaceholder = html.Element.tag('flt-semantics-placeholder');
+    final html.Element placeholder = _semanticsPlaceholder = html.Element.tag('flt-semantics-placeholder');
 
     // Only listen to "click" because other kinds of events are reported via
     // PointerBinding.
-    _semanticsPlaceholder.addEventListener('click', (html.Event event) {
+    placeholder.addEventListener('click', (html.Event event) {
       tryEnableSemantics(event);
     }, true);
 
-    _semanticsPlaceholder
+    placeholder
       ..setAttribute('role', 'button')
       ..setAttribute('aria-label', placeholderMessage);
-    _semanticsPlaceholder.style
+    placeholder.style
       ..position = 'absolute'
       ..left = '0'
       ..top = '0'
       ..right = '0'
       ..bottom = '0';
 
-    return _semanticsPlaceholder;
+    return placeholder;
   }
 }
diff --git a/lib/web_ui/lib/src/engine/semantics/tappable.dart b/lib/web_ui/lib/src/engine/semantics/tappable.dart
index e60c9881918e5..cd416e106cf82 100644
--- a/lib/web_ui/lib/src/engine/semantics/tappable.dart
+++ b/lib/web_ui/lib/src/engine/semantics/tappable.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Listens to HTML "click" gestures detected by the browser.
@@ -15,7 +15,7 @@ class Tappable extends RoleManager {
   Tappable(SemanticsObject semanticsObject)
       : super(Role.tappable, semanticsObject);
 
-  html.EventListener _clickListener;
+  html.EventListener? _clickListener;
 
   @override
   void update() {
diff --git a/lib/web_ui/lib/src/engine/semantics/text_field.dart b/lib/web_ui/lib/src/engine/semantics/text_field.dart
index 5018d8d2d5aa2..bc5bf16116e5a 100644
--- a/lib/web_ui/lib/src/engine/semantics/text_field.dart
+++ b/lib/web_ui/lib/src/engine/semantics/text_field.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Text editing used by accesibility mode.
@@ -46,11 +46,10 @@ class SemanticsTextEditingStrategy extends DefaultTextEditingStrategy {
 
   @override
   void initializeTextEditing(InputConfiguration inputConfig,
-      {_OnChangeCallback onChange, _OnActionCallback onAction}) {
+      {_OnChangeCallback? onChange, _OnActionCallback? onAction}) {
     // In accesibilty mode, the user of this class is supposed to insert the
     // [domElement] on their own. Let's make sure they did.
-    assert(domElement != null);
-    assert(html.document.body.contains(domElement));
+    assert(html.document.body!.contains(domElement));
 
     isEnabled = true;
     _inputConfiguration = inputConfig;
@@ -61,7 +60,7 @@ class SemanticsTextEditingStrategy extends DefaultTextEditingStrategy {
   }
 
   @override
-  void setEditingState(EditingState editingState) {
+  void setEditingState(EditingState? editingState) {
     super.setEditingState(editingState);
 
     // Refocus after setting editing state.
@@ -91,8 +90,8 @@ class TextField extends RoleManager {
     _setupDomElement();
   }
 
-  SemanticsTextEditingStrategy textEditingElement;
-  html.Element get _textFieldElement => textEditingElement.domElement;
+  SemanticsTextEditingStrategy? textEditingElement;
+  html.Element get _textFieldElement => textEditingElement!.domElement;
 
   void _setupDomElement() {
     // On iOS, even though the semantic text field is transparent, the cursor
@@ -119,8 +118,8 @@ class TextField extends RoleManager {
       //   and size of the parent `<flt-semantics>` element.
       ..top = '0'
       ..left = '0'
-      ..width = '${semanticsObject.rect.width}px'
-      ..height = '${semanticsObject.rect.height}px';
+      ..width = '${semanticsObject.rect!.width}px'
+      ..height = '${semanticsObject.rect!.height}px';
     semanticsObject.element.append(_textFieldElement);
 
     switch (browserEngine) {
@@ -160,18 +159,18 @@ class TextField extends RoleManager {
   /// events are present regardless of whether accessibility is enabled or not,
   /// this mode is always enabled.
   void _initializeForWebkit() {
-    num lastTouchStartOffsetX;
-    num lastTouchStartOffsetY;
+    num? lastTouchStartOffsetX;
+    num? lastTouchStartOffsetY;
 
     _textFieldElement.addEventListener('touchstart', (html.Event event) {
       textEditing.useCustomEditableElement(textEditingElement);
-      final html.TouchEvent touchEvent = event;
+      final html.TouchEvent touchEvent = event as html.TouchEvent;
       lastTouchStartOffsetX = touchEvent.changedTouches.last.client.x;
       lastTouchStartOffsetY = touchEvent.changedTouches.last.client.y;
     }, true);
 
     _textFieldElement.addEventListener('touchend', (html.Event event) {
-      final html.TouchEvent touchEvent = event;
+      final html.TouchEvent touchEvent = event as html.TouchEvent;
 
       if (lastTouchStartOffsetX != null) {
         assert(lastTouchStartOffsetY != null);
diff --git a/lib/web_ui/lib/src/engine/services/buffers.dart b/lib/web_ui/lib/src/engine/services/buffers.dart
index aff4c83454082..c8778a003e21b 100644
--- a/lib/web_ui/lib/src/engine/services/buffers.dart
+++ b/lib/web_ui/lib/src/engine/services/buffers.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 abstract class _TypedDataBuffer<E> extends ListBase<E> {
@@ -88,7 +88,7 @@ abstract class _TypedDataBuffer<E> extends ListBase<E> {
   /// least [start] elements, and if [end] is specified, it must be greater than
   /// or equal to [start] and [values] must have at least [end] elements.
   @override
-  void addAll(Iterable<E> values, [int start = 0, int end]) {
+  void addAll(Iterable<E> values, [int start = 0, int? end]) {
     RangeError.checkNotNegative(start, 'start');
     if (end != null && start > end) {
       throw RangeError.range(end, start, null, 'end');
@@ -107,7 +107,7 @@ abstract class _TypedDataBuffer<E> extends ListBase<E> {
   /// least [start] elements, and if [end] is specified, it must be greater than
   /// or equal to [start] and [values] must have at least [end] elements.
   @override
-  void insertAll(int index, Iterable<E> values, [int start = 0, int end]) {
+  void insertAll(int index, Iterable<E> values, [int start = 0, int? end]) {
     RangeError.checkValidIndex(index, this, 'index', _length + 1);
     RangeError.checkNotNegative(start, 'start');
     if (end != null) {
@@ -167,11 +167,11 @@ abstract class _TypedDataBuffer<E> extends ListBase<E> {
   }
 
   // Reverses the range [start..end) of buffer.
-  static void _reverse(List<Object> buffer, int start, int end) {
+  static void _reverse(List<Object?> buffer, int start, int end) {
     end--; // Point to last element, not after last element.
     while (start < end) {
-      final Object first = buffer[start];
-      final Object last = buffer[end];
+      final Object? first = buffer[start];
+      final Object? last = buffer[end];
       buffer[end] = first;
       buffer[start] = last;
       start++;
@@ -184,8 +184,8 @@ abstract class _TypedDataBuffer<E> extends ListBase<E> {
   /// This allows [addAll] and [insertAll] to share implementation without a
   /// subclass unexpectedly overriding both when it intended to only override
   /// [addAll].
-  void _addAll(Iterable<E> values, [int start = 0, int end]) {
-    if (values is List) {
+  void _addAll(Iterable<E> values, [int start = 0, int? end]) {
+    if (values is List<E>) {
       end ??= values.length;
     }
 
@@ -212,13 +212,10 @@ abstract class _TypedDataBuffer<E> extends ListBase<E> {
 
   /// Like [insertAll], but with a guaranteed non-`null` [start] and [end].
   void _insertKnownLength(int index, Iterable<E> values, int start, int end) {
-    if (values is List) {
-      end ??= values.length;
-      if (start > values.length || end > values.length) {
-        throw StateError('Too few elements');
-      }
-    } else {
-      assert(end != null);
+    assert(values != null); // ignore: unnecessary_null_comparison
+    assert(end != null); // ignore: unnecessary_null_comparison
+    if (start > values.length || end > values.length) {
+      throw StateError('Too few elements');
     }
 
     final int valuesLength = end - start;
@@ -268,7 +265,7 @@ abstract class _TypedDataBuffer<E> extends ListBase<E> {
   /// be. If [requiredCapacity] is not null, it will be at least that
   /// size. It will always have at least have double the capacity of
   /// the current buffer.
-  List<E> _createBiggerBuffer(int requiredCapacity) {
+  List<E> _createBiggerBuffer(int? requiredCapacity) {
     int newLength = _buffer.length * 2;
     if (requiredCapacity != null && newLength < requiredCapacity) {
       newLength = requiredCapacity;
diff --git a/lib/web_ui/lib/src/engine/services/message_codec.dart b/lib/web_ui/lib/src/engine/services/message_codec.dart
index 0784135b145ca..d4e4a31941bff 100644
--- a/lib/web_ui/lib/src/engine/services/message_codec.dart
+++ b/lib/web_ui/lib/src/engine/services/message_codec.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A message encoding/decoding mechanism.
@@ -18,7 +18,7 @@ abstract class MessageCodec<T> {
   /// Encodes the specified [message] in binary.
   ///
   /// Returns null if the message is null.
-  ByteData encodeMessage(T message);
+  ByteData? encodeMessage(T message);
 
   /// Decodes the specified [message] from binary.
   ///
@@ -31,6 +31,7 @@ abstract class MessageCodec<T> {
 class MethodCall {
   /// Creates a [MethodCall] representing the invocation of [method] with the
   /// specified [arguments].
+  // ignore: unnecessary_null_comparison
   const MethodCall(this.method, [this.arguments]) : assert(method != null);
 
   /// The name of the method to be called.
@@ -57,10 +58,10 @@ class MethodCall {
 ///   between Flutter and platform plugins.
 abstract class MethodCodec {
   /// Encodes the specified [methodCall] into binary.
-  ByteData encodeMethodCall(MethodCall methodCall);
+  ByteData? encodeMethodCall(MethodCall methodCall);
 
   /// Decodes the specified [methodCall] from binary.
-  MethodCall decodeMethodCall(ByteData methodCall);
+  MethodCall decodeMethodCall(ByteData? methodCall);
 
   /// Decodes the specified result [envelope] from binary.
   ///
@@ -69,14 +70,14 @@ abstract class MethodCodec {
   dynamic decodeEnvelope(ByteData envelope);
 
   /// Encodes a successful [result] into a binary envelope.
-  ByteData encodeSuccessEnvelope(dynamic result);
+  ByteData? encodeSuccessEnvelope(dynamic result);
 
   /// Encodes an error result into a binary envelope.
   ///
   /// The specified error [code], human-readable error [message], and error
   /// [details] correspond to the fields of [PlatformException].
-  ByteData encodeErrorEnvelope(
-      {@required String code, String message, dynamic details});
+  ByteData? encodeErrorEnvelope(
+      {required String code, String? message, dynamic details});
 }
 
 /// Thrown to indicate that a platform interaction failed in the platform
@@ -97,16 +98,16 @@ class PlatformException implements Exception {
   /// [message], and with the optional error [details] which must be a valid
   /// value for the [MethodCodec] involved in the interaction.
   PlatformException({
-    @required this.code,
+    required this.code,
     this.message,
     this.details,
-  }) : assert(code != null);
+  }) : assert(code != null); // ignore: unnecessary_null_comparison
 
   /// An error code.
   final String code;
 
   /// A human-readable error message, possibly null.
-  final String message;
+  final String? message;
 
   /// Error details, possibly null.
   final dynamic details;
@@ -131,7 +132,7 @@ class MissingPluginException implements Exception {
   MissingPluginException([this.message]);
 
   /// A human-readable error message, possibly null.
-  final String message;
+  final String? message;
 
   @override
   String toString() => 'MissingPluginException($message)';
diff --git a/lib/web_ui/lib/src/engine/services/message_codecs.dart b/lib/web_ui/lib/src/engine/services/message_codecs.dart
index 42537f7106694..9f85cb95ecf4f 100644
--- a/lib/web_ui/lib/src/engine/services/message_codecs.dart
+++ b/lib/web_ui/lib/src/engine/services/message_codecs.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// [MessageCodec] with unencoded binary messages represented using [ByteData].
@@ -31,17 +31,11 @@ class StringCodec implements MessageCodec<String> {
 
   @override
   String decodeMessage(ByteData message) {
-    if (message == null) {
-      return null;
-    }
     return utf8.decoder.convert(message.buffer.asUint8List());
   }
 
   @override
   ByteData encodeMessage(String message) {
-    if (message == null) {
-      return null;
-    }
     final Uint8List encoded = utf8.encoder.convert(message);
     return encoded.buffer.asByteData();
   }
@@ -74,7 +68,7 @@ class JSONMessageCodec implements MessageCodec<dynamic> {
   const JSONMessageCodec();
 
   @override
-  ByteData encodeMessage(dynamic message) {
+  ByteData? encodeMessage(dynamic message) {
     if (message == null) {
       return null;
     }
@@ -82,7 +76,7 @@ class JSONMessageCodec implements MessageCodec<dynamic> {
   }
 
   @override
-  dynamic decodeMessage(ByteData message) {
+  dynamic decodeMessage(ByteData? message) {
     if (message == null) {
       return message;
     }
@@ -113,7 +107,7 @@ class JSONMethodCodec implements MethodCodec {
   const JSONMethodCodec();
 
   @override
-  ByteData encodeMethodCall(MethodCall call) {
+  ByteData? encodeMethodCall(MethodCall call) {
     return const JSONMessageCodec().encodeMessage(<String, dynamic>{
       'method': call.method,
       'args': call.arguments,
@@ -121,7 +115,7 @@ class JSONMethodCodec implements MethodCodec {
   }
 
   @override
-  MethodCall decodeMethodCall(ByteData methodCall) {
+  MethodCall decodeMethodCall(ByteData? methodCall) {
     final dynamic decoded = const JSONMessageCodec().decodeMessage(methodCall);
     if (decoded is! Map) {
       throw FormatException('Expected method call Map, got $decoded');
@@ -156,14 +150,14 @@ class JSONMethodCodec implements MethodCodec {
   }
 
   @override
-  ByteData encodeSuccessEnvelope(dynamic result) {
+  ByteData? encodeSuccessEnvelope(dynamic result) {
     return const JSONMessageCodec().encodeMessage(<dynamic>[result]);
   }
 
   @override
-  ByteData encodeErrorEnvelope(
-      {@required String code, String message, dynamic details}) {
-    assert(code != null);
+  ByteData? encodeErrorEnvelope(
+      {required String code, String? message, dynamic details}) {
+    assert(code != null); // ignore: unnecessary_null_comparison
     return const JSONMessageCodec()
         .encodeMessage(<dynamic>[code, message, details]);
   }
@@ -271,7 +265,7 @@ class StandardMessageCodec implements MessageCodec<dynamic> {
   const StandardMessageCodec();
 
   @override
-  ByteData encodeMessage(dynamic message) {
+  ByteData? encodeMessage(dynamic message) {
     if (message == null) {
       return null;
     }
@@ -281,7 +275,7 @@ class StandardMessageCodec implements MessageCodec<dynamic> {
   }
 
   @override
-  dynamic decodeMessage(ByteData message) {
+  dynamic decodeMessage(ByteData? message) {
     if (message == null) {
       return null;
     }
@@ -326,7 +320,7 @@ class StandardMessageCodec implements MessageCodec<dynamic> {
       buffer.putUint8(_valueString);
       final List<int> bytes = utf8.encoder.convert(value);
       writeSize(buffer, bytes.length);
-      buffer.putUint8List(bytes);
+      buffer.putUint8List(bytes as Uint8List);
     } else if (value is Uint8List) {
       buffer.putUint8(_valueUint8List);
       writeSize(buffer, value.length);
@@ -428,9 +422,9 @@ class StandardMessageCodec implements MessageCodec<dynamic> {
         break;
       case _valueList:
         final int length = readSize(buffer);
-        result = List<dynamic>(length);
+        result = <dynamic>[];
         for (int i = 0; i < length; i++) {
-          result[i] = readValue(buffer);
+          result.add(readValue(buffer));
         }
         break;
       case _valueMap:
@@ -517,8 +511,8 @@ class StandardMethodCodec implements MethodCodec {
   }
 
   @override
-  MethodCall decodeMethodCall(ByteData methodCall) {
-    final ReadBuffer buffer = ReadBuffer(methodCall);
+  MethodCall decodeMethodCall(ByteData? methodCall) {
+    final ReadBuffer buffer = ReadBuffer(methodCall!);
     final dynamic method = messageCodec.readValue(buffer);
     final dynamic arguments = messageCodec.readValue(buffer);
     if (method is String && !buffer.hasRemaining)
@@ -537,7 +531,7 @@ class StandardMethodCodec implements MethodCodec {
 
   @override
   ByteData encodeErrorEnvelope(
-      {@required String code, String message, dynamic details}) {
+      {required String code, String? message, dynamic details}) {
     final WriteBuffer buffer = WriteBuffer();
     buffer.putUint8(1);
     messageCodec.writeValue(buffer, code);
diff --git a/lib/web_ui/lib/src/engine/services/serialization.dart b/lib/web_ui/lib/src/engine/services/serialization.dart
index b38ae180976b9..431864a25eb73 100644
--- a/lib/web_ui/lib/src/engine/services/serialization.dart
+++ b/lib/web_ui/lib/src/engine/services/serialization.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
 /// Write-only buffer for incrementally building a [ByteData] instance.
@@ -13,47 +12,57 @@ part of engine;
 /// The byte order used is [Endian.host] throughout.
 class WriteBuffer {
   /// Creates an interface for incrementally building a [ByteData] instance.
-  WriteBuffer() {
-    _buffer = Uint8Buffer();
-    _eightBytes = ByteData(8);
-    _eightBytesAsList = _eightBytes.buffer.asUint8List();
+  factory WriteBuffer() {
+    final Uint8Buffer buffer = Uint8Buffer();
+    final ByteData eightBytes = ByteData(8);
+    final Uint8List eightBytesAsList = eightBytes.buffer.asUint8List();
+    return WriteBuffer._(buffer, eightBytes, eightBytesAsList);
   }
 
-  Uint8Buffer _buffer;
-  ByteData _eightBytes;
-  Uint8List _eightBytesAsList;
+  WriteBuffer._(this._buffer, this._eightBytes, this._eightBytesAsList);
+
+  bool _debugFinalized = false;
+  final Uint8Buffer _buffer;
+  final ByteData _eightBytes;
+  final Uint8List _eightBytesAsList;
 
   /// Write a Uint8 into the buffer.
   void putUint8(int byte) {
+    assert(!_debugFinalized);
     _buffer.add(byte);
   }
 
   /// Write a Uint16 into the buffer.
   void putUint16(int value) {
+    assert(!_debugFinalized);
     _eightBytes.setUint16(0, value, Endian.host);
     _buffer.addAll(_eightBytesAsList, 0, 2);
   }
 
   /// Write a Uint32 into the buffer.
   void putUint32(int value) {
+    assert(!_debugFinalized);
     _eightBytes.setUint32(0, value, Endian.host);
     _buffer.addAll(_eightBytesAsList, 0, 4);
   }
 
   /// Write an Int32 into the buffer.
   void putInt32(int value) {
+    assert(!_debugFinalized);
     _eightBytes.setInt32(0, value, Endian.host);
     _buffer.addAll(_eightBytesAsList, 0, 4);
   }
 
   /// Write an Int64 into the buffer.
   void putInt64(int value) {
+    assert(!_debugFinalized);
     _eightBytes.setInt64(0, value, Endian.host);
     _buffer.addAll(_eightBytesAsList, 0, 8);
   }
 
   /// Write an Float64 into the buffer.
   void putFloat64(double value) {
+    assert(!_debugFinalized);
     _alignTo(8);
     _eightBytes.setFloat64(0, value, Endian.host);
     _buffer.addAll(_eightBytesAsList);
@@ -61,11 +70,13 @@ class WriteBuffer {
 
   /// Write all the values from a [Uint8List] into the buffer.
   void putUint8List(Uint8List list) {
+    assert(!_debugFinalized);
     _buffer.addAll(list);
   }
 
   /// Write all the values from a [Int32List] into the buffer.
   void putInt32List(Int32List list) {
+    assert(!_debugFinalized);
     _alignTo(4);
     _buffer
         .addAll(list.buffer.asUint8List(list.offsetInBytes, 4 * list.length));
@@ -73,6 +84,7 @@ class WriteBuffer {
 
   /// Write all the values from an [Int64List] into the buffer.
   void putInt64List(Int64List list) {
+    assert(!_debugFinalized);
     _alignTo(8);
     _buffer
         .addAll(list.buffer.asUint8List(list.offsetInBytes, 8 * list.length));
@@ -80,6 +92,7 @@ class WriteBuffer {
 
   /// Write all the values from a [Float64List] into the buffer.
   void putFloat64List(Float64List list) {
+    assert(!_debugFinalized);
     _alignTo(8);
     _buffer
         .addAll(list.buffer.asUint8List(list.offsetInBytes, 8 * list.length));
@@ -96,8 +109,8 @@ class WriteBuffer {
 
   /// Finalize and return the written [ByteData].
   ByteData done() {
+    _debugFinalized = true;
     final ByteData result = _buffer.buffer.asByteData(0, _buffer.lengthInBytes);
-    _buffer = null;
     return result;
   }
 }
@@ -107,7 +120,7 @@ class WriteBuffer {
 /// The byte order used is [Endian.host] throughout.
 class ReadBuffer {
   /// Creates a [ReadBuffer] for reading from the specified [data].
-  ReadBuffer(this.data) : assert(data != null);
+  ReadBuffer(this.data) : assert(data != null); // ignore: unnecessary_null_comparison
 
   /// The underlying data being read.
   final ByteData data;
diff --git a/lib/web_ui/lib/src/engine/shader.dart b/lib/web_ui/lib/src/engine/shader.dart
index d642d6c3f31c2..cdce61daf2bb5 100644
--- a/lib/web_ui/lib/src/engine/shader.dart
+++ b/lib/web_ui/lib/src/engine/shader.dart
@@ -2,25 +2,25 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 bool _offsetIsValid(ui.Offset offset) {
-  assert(offset != null, 'Offset argument was null.');
+  assert(offset != null, 'Offset argument was null.'); // ignore: unnecessary_null_comparison
   assert(!offset.dx.isNaN && !offset.dy.isNaN,
       'Offset argument contained a NaN value.');
   return true;
 }
 
 bool _matrix4IsValid(Float32List matrix4) {
-  assert(matrix4 != null, 'Matrix4 argument was null.');
+  assert(matrix4 != null, 'Matrix4 argument was null.'); // ignore: unnecessary_null_comparison
   assert(matrix4.length == 16, 'Matrix4 must have 16 entries.');
   return true;
 }
 
 abstract class EngineShader {
   /// Create a shader for use in the Skia backend.
-  js.JsObject createSkiaShader();
+  js.JsObject? createSkiaShader();
 }
 
 abstract class EngineGradient implements ui.Gradient, EngineShader {
@@ -28,7 +28,7 @@ abstract class EngineGradient implements ui.Gradient, EngineShader {
   EngineGradient._();
 
   /// Creates a fill style to be used in painting.
-  Object createPaintStyle(html.CanvasRenderingContext2D ctx);
+  Object createPaintStyle(html.CanvasRenderingContext2D? ctx);
 
   List<dynamic> webOnlySerializeToCssPaint() {
     throw UnsupportedError('CSS paint not implemented for this shader type');
@@ -39,10 +39,10 @@ class GradientSweep extends EngineGradient {
   GradientSweep(this.center, this.colors, this.colorStops, this.tileMode,
       this.startAngle, this.endAngle, this.matrix4)
       : assert(_offsetIsValid(center)),
-        assert(colors != null),
-        assert(tileMode != null),
-        assert(startAngle != null),
-        assert(endAngle != null),
+        assert(colors != null), // ignore: unnecessary_null_comparison
+        assert(tileMode != null), // ignore: unnecessary_null_comparison
+        assert(startAngle != null), // ignore: unnecessary_null_comparison
+        assert(endAngle != null), // ignore: unnecessary_null_comparison
         assert(startAngle < endAngle),
         assert(matrix4 == null || _matrix4IsValid(matrix4)),
         super._() {
@@ -50,17 +50,17 @@ class GradientSweep extends EngineGradient {
   }
 
   @override
-  Object createPaintStyle(html.CanvasRenderingContext2D ctx) {
+  Object createPaintStyle(html.CanvasRenderingContext2D? ctx) {
     throw UnimplementedError();
   }
 
   final ui.Offset center;
   final List<ui.Color> colors;
-  final List<double> colorStops;
+  final List<double>? colorStops;
   final ui.TileMode tileMode;
   final double startAngle;
   final double endAngle;
-  final Float32List matrix4;
+  final Float32List? matrix4;
 
   @override
   js.JsObject createSkiaShader() {
@@ -68,7 +68,7 @@ class GradientSweep extends EngineGradient {
   }
 }
 
-void _validateColorStops(List<ui.Color> colors, List<double> colorStops) {
+void _validateColorStops(List<ui.Color> colors, List<double>? colorStops) {
   if (colorStops == null) {
     if (colors.length != 2)
       throw ArgumentError(
@@ -87,11 +87,11 @@ class GradientLinear extends EngineGradient {
     this.colors,
     this.colorStops,
     this.tileMode,
-    Float64List matrix,
+    Float64List? matrix,
   )   : assert(_offsetIsValid(from)),
         assert(_offsetIsValid(to)),
-        assert(colors != null),
-        assert(tileMode != null),
+        assert(colors != null), // ignore: unnecessary_null_comparison
+        assert(tileMode != null), // ignore: unnecessary_null_comparison
         this.matrix4 = matrix == null ? null : _FastMatrix64(matrix),
         super._() {
     if (assertionsEnabled) {
@@ -102,35 +102,36 @@ class GradientLinear extends EngineGradient {
   final ui.Offset from;
   final ui.Offset to;
   final List<ui.Color> colors;
-  final List<double> colorStops;
+  final List<double>? colorStops;
   final ui.TileMode tileMode;
-  final _FastMatrix64 matrix4;
+  final _FastMatrix64? matrix4;
 
   @override
-  html.CanvasGradient createPaintStyle(html.CanvasRenderingContext2D ctx) {
-    final bool hasMatrix = matrix4 != null;
+  html.CanvasGradient createPaintStyle(html.CanvasRenderingContext2D? ctx) {
+    _FastMatrix64? matrix4 = this.matrix4;
     html.CanvasGradient gradient;
-    if (hasMatrix) {
+    if (matrix4 != null) {
       final centerX = (from.dx + to.dx) / 2.0;
       final centerY = (from.dy + to.dy) / 2.0;
       matrix4.transform(from.dx - centerX, from.dy - centerY);
       final double fromX = matrix4.transformedX + centerX;
       final double fromY = matrix4.transformedY + centerY;
       matrix4.transform(to.dx - centerX, to.dy - centerY);
-      gradient = ctx.createLinearGradient(fromX, fromY,
+      gradient = ctx!.createLinearGradient(fromX, fromY,
           matrix4.transformedX + centerX, matrix4.transformedY + centerY);
     } else {
-      gradient = ctx.createLinearGradient(from.dx, from.dy, to.dx, to.dy);
+      gradient = ctx!.createLinearGradient(from.dx, from.dy, to.dx, to.dy);
     }
 
+    final List<double>? colorStops = this.colorStops;
     if (colorStops == null) {
       assert(colors.length == 2);
-      gradient.addColorStop(0, colorToCssString(colors[0]));
-      gradient.addColorStop(1, colorToCssString(colors[1]));
+      gradient.addColorStop(0, colorToCssString(colors[0])!);
+      gradient.addColorStop(1, colorToCssString(colors[1])!);
       return gradient;
     }
     for (int i = 0; i < colors.length; i++) {
-      gradient.addColorStop(colorStops[i], colorToCssString(colors[i]));
+      gradient.addColorStop(colorStops[i], colorToCssString(colors[i])!);
     }
     return gradient;
   }
@@ -154,7 +155,7 @@ class GradientLinear extends EngineGradient {
   }
 
   @override
-  js.JsObject createSkiaShader() {
+  js.JsObject? createSkiaShader() {
     assert(experimentalUseSkia);
 
     var jsColors = makeColorList(colors);
@@ -181,35 +182,36 @@ class GradientRadial extends EngineGradient {
   final ui.Offset center;
   final double radius;
   final List<ui.Color> colors;
-  final List<double> colorStops;
+  final List<double>? colorStops;
   final ui.TileMode tileMode;
-  final Float32List matrix4;
+  final Float32List? matrix4;
 
   @override
-  Object createPaintStyle(html.CanvasRenderingContext2D ctx) {
+  Object createPaintStyle(html.CanvasRenderingContext2D? ctx) {
     if (!experimentalUseSkia) {
       if (tileMode != ui.TileMode.clamp) {
         throw UnimplementedError(
             'TileMode not supported in GradientRadial shader');
       }
     }
-    final html.CanvasGradient gradient = ctx.createRadialGradient(
+    final html.CanvasGradient gradient = ctx!.createRadialGradient(
         center.dx, center.dy, 0, center.dx, center.dy, radius);
+    final List<double>? colorStops = this.colorStops;
     if (colorStops == null) {
       assert(colors.length == 2);
-      gradient.addColorStop(0, colorToCssString(colors[0]));
-      gradient.addColorStop(1, colorToCssString(colors[1]));
+      gradient.addColorStop(0, colorToCssString(colors[0])!);
+      gradient.addColorStop(1, colorToCssString(colors[1])!);
       return gradient;
     } else {
       for (int i = 0; i < colors.length; i++) {
-        gradient.addColorStop(colorStops[i], colorToCssString(colors[i]));
+        gradient.addColorStop(colorStops[i], colorToCssString(colors[i])!);
       }
     }
     return gradient;
   }
 
   @override
-  js.JsObject createSkiaShader() {
+  js.JsObject? createSkiaShader() {
     assert(experimentalUseSkia);
 
     var jsColors = makeColorList(colors);
@@ -236,17 +238,17 @@ class GradientConical extends EngineGradient {
   final ui.Offset center;
   final double radius;
   final List<ui.Color> colors;
-  final List<double> colorStops;
+  final List<double>? colorStops;
   final ui.TileMode tileMode;
-  final Float32List matrix4;
+  final Float32List? matrix4;
 
   @override
-  Object createPaintStyle(html.CanvasRenderingContext2D ctx) {
+  Object createPaintStyle(html.CanvasRenderingContext2D? ctx) {
     throw UnimplementedError();
   }
 
   @override
-  js.JsObject createSkiaShader() {
+  js.JsObject? createSkiaShader() {
     assert(experimentalUseSkia);
 
     var jsColors = makeColorList(colors);
@@ -293,7 +295,7 @@ class EngineImageFilter implements ui.ImageFilter {
   }
 }
 
-js.JsObject _skTileMode(ui.TileMode tileMode) {
+js.JsObject? _skTileMode(ui.TileMode tileMode) {
   switch (tileMode) {
     case ui.TileMode.clamp:
       return canvasKit['TileMode']['Clamp'];
@@ -316,6 +318,6 @@ class EngineImageShader implements ui.ImageShader, EngineShader {
   final Float64List matrix4;
   final SkImage _skImage;
 
-  js.JsObject createSkiaShader() => _skImage.skImage.callMethod(
+  js.JsObject? createSkiaShader() => _skImage.skImage!.callMethod(
       'makeShader', <dynamic>[_skTileMode(tileModeX), _skTileMode(tileModeY)]);
 }
diff --git a/lib/web_ui/lib/src/engine/shadow.dart b/lib/web_ui/lib/src/engine/shadow.dart
index c5bebaf7fa350..a106eaa78304f 100644
--- a/lib/web_ui/lib/src/engine/shadow.dart
+++ b/lib/web_ui/lib/src/engine/shadow.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// How far is the light source from the surface of the UI.
@@ -81,8 +81,8 @@ ui.Rect computePenumbraBounds(ui.Rect shape, double elevation) {
 @immutable
 class SurfaceShadowData {
   const SurfaceShadowData({
-    @required this.blurWidth,
-    @required this.offset,
+    required this.blurWidth,
+    required this.offset,
   });
 
   /// The length in pixels of the shadow.
@@ -106,7 +106,7 @@ class SurfaceShadowData {
 /// (cannot) use Skia's shadow API directly. However, this algorithms is
 /// consistent with [computePenumbraBounds] used by [RecordingCanvas] during
 /// bounds estimation.
-SurfaceShadowData computeShadow(ui.Rect shape, double elevation) {
+SurfaceShadowData? computeShadow(ui.Rect shape, double elevation) {
   if (elevation == 0.0) {
     return null;
   }
@@ -128,13 +128,13 @@ SurfaceShadowData computeShadow(ui.Rect shape, double elevation) {
 
 /// Applies a CSS shadow to the [shape].
 void applyCssShadow(
-    html.Element element, ui.Rect shape, double elevation, ui.Color color) {
-  final SurfaceShadowData shadow = computeShadow(shape, elevation);
+    html.Element? element, ui.Rect shape, double elevation, ui.Color color) {
+  final SurfaceShadowData? shadow = computeShadow(shape, elevation);
   if (shadow == null) {
-    element.style.boxShadow = 'none';
+    element!.style.boxShadow = 'none';
   } else {
     color = toShadowColor(color);
-    element.style.boxShadow = '${shadow.offset.dx}px ${shadow.offset.dy}px '
+    element!.style.boxShadow = '${shadow.offset.dx}px ${shadow.offset.dy}px '
         '${shadow.blurWidth}px 0px rgba(${color.red}, ${color.green}, ${color.blue}, ${color.alpha / 255})';
   }
 }
diff --git a/lib/web_ui/lib/src/engine/surface/backdrop_filter.dart b/lib/web_ui/lib/src/engine/surface/backdrop_filter.dart
index 6ed5b2356d80a..ca5644af2c3d2 100644
--- a/lib/web_ui/lib/src/engine/surface/backdrop_filter.dart
+++ b/lib/web_ui/lib/src/engine/surface/backdrop_filter.dart
@@ -2,13 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A surface that applies an image filter to background.
 class PersistedBackdropFilter extends PersistedContainerSurface
     implements ui.BackdropFilterEngineLayer {
-  PersistedBackdropFilter(PersistedBackdropFilter oldLayer, this.filter)
+  PersistedBackdropFilter(PersistedBackdropFilter? oldLayer, this.filter)
       : super(oldLayer);
 
   final EngineImageFilter filter;
@@ -17,14 +17,14 @@ class PersistedBackdropFilter extends PersistedContainerSurface
   /// [rootElement] is used to host child in front of [filterElement] that
   /// is transformed to cover background.
   @override
-  html.Element get childContainer => _childContainer;
-  html.Element _childContainer;
-  html.Element _filterElement;
-  ui.Rect _activeClipBounds;
+  html.Element? get childContainer => _childContainer;
+  html.Element? _childContainer;
+  html.Element? _filterElement;
+  ui.Rect? _activeClipBounds;
   // Cached inverted transform for _transform.
-  Matrix4 _invertedTransform;
+  late Matrix4 _invertedTransform;
   // Reference to transform last used to cache [_invertedTransform].
-  Matrix4 _previousTransform;
+  Matrix4? _previousTransform;
 
   @override
   void adoptElements(PersistedBackdropFilter oldSurface) {
@@ -39,14 +39,14 @@ class PersistedBackdropFilter extends PersistedContainerSurface
     final html.Element element = defaultCreateElement('flt-backdrop')
       ..style.transformOrigin = '0 0 0';
     _childContainer = html.Element.tag('flt-backdrop-interior');
-    _childContainer.style.position = 'absolute';
+    _childContainer!.style.position = 'absolute';
     if (_debugExplainSurfaceStats) {
       // This creates an additional interior element. Count it too.
       _surfaceStatsFor(this).allocatedDomNodeCount++;
     }
     _filterElement = defaultCreateElement('flt-backdrop-filter');
-    _filterElement.style.transformOrigin = '0 0 0';
-    element..append(_filterElement)..append(_childContainer);
+    _filterElement!.style.transformOrigin = '0 0 0';
+    element..append(_filterElement!)..append(_childContainer!);
     return element;
   }
 
@@ -63,7 +63,7 @@ class PersistedBackdropFilter extends PersistedContainerSurface
   @override
   void apply() {
     if (_previousTransform != _transform) {
-      _invertedTransform = Matrix4.inverted(_transform);
+      _invertedTransform = Matrix4.inverted(_transform!);
       _previousTransform = _transform;
     }
     // https://api.flutter.dev/flutter/widgets/BackdropFilter-class.html
@@ -82,19 +82,19 @@ class PersistedBackdropFilter extends PersistedContainerSurface
     double top = rect.top;
     double width = rect.width;
     double height = rect.height;
-    PersistedContainerSurface parentSurface = parent;
+    PersistedContainerSurface? parentSurface = parent;
     while (parentSurface != null) {
       if (parentSurface.isClipping) {
-        _activeClipBounds = parentSurface._localClipBounds;
-        left = _activeClipBounds.left;
-        top = _activeClipBounds.top;
-        width = _activeClipBounds.width;
-        height = _activeClipBounds.height;
+        final ui.Rect activeClipBounds = (_activeClipBounds = parentSurface._localClipBounds)!;
+        left = activeClipBounds.left;
+        top = activeClipBounds.top;
+        width = activeClipBounds.width;
+        height = activeClipBounds.height;
         break;
       }
       parentSurface = parentSurface.parent;
     }
-    final html.CssStyleDeclaration filterElementStyle = _filterElement.style;
+    final html.CssStyleDeclaration filterElementStyle = _filterElement!.style;
     filterElementStyle
       ..position = 'absolute'
       ..left = '${left}px'
@@ -113,10 +113,10 @@ class PersistedBackdropFilter extends PersistedContainerSurface
       // Gaussian blur with standard deviation (normal distribution),
       // the blur will fall within 2 * sigma pixels.
       if (browserEngine == BrowserEngine.webkit) {
-        domRenderer.setElementStyle(_filterElement, '-webkit-backdrop-filter',
+        domRenderer.setElementStyle(_filterElement!, '-webkit-backdrop-filter',
             _imageFilterToCss(filter));
       }
-      domRenderer.setElementStyle(_filterElement, 'backdrop-filter', _imageFilterToCss(filter));
+      domRenderer.setElementStyle(_filterElement!, 'backdrop-filter', _imageFilterToCss(filter));
     }
   }
 
@@ -132,7 +132,7 @@ class PersistedBackdropFilter extends PersistedContainerSurface
 
   void _checkForUpdatedAncestorClipElement() {
     // If parent clip element has moved, adjust bounds.
-    PersistedContainerSurface parentSurface = parent;
+    PersistedContainerSurface? parentSurface = parent;
     while (parentSurface != null) {
       if (parentSurface.isClipping) {
         if (parentSurface._localClipBounds != _activeClipBounds) {
diff --git a/lib/web_ui/lib/src/engine/surface/canvas.dart b/lib/web_ui/lib/src/engine/surface/canvas.dart
index eff7ec956b87f..a180cac9a3dec 100644
--- a/lib/web_ui/lib/src/engine/surface/canvas.dart
+++ b/lib/web_ui/lib/src/engine/surface/canvas.dart
@@ -2,30 +2,30 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
 class SurfaceCanvas implements ui.Canvas {
-  RecordingCanvas/*!*/ _canvas;
+  RecordingCanvas _canvas;
 
-  SurfaceCanvas(EnginePictureRecorder recorder, [ui.Rect cullRect])
-      : assert(recorder != null) {
+  factory SurfaceCanvas(EnginePictureRecorder recorder, [ui.Rect? cullRect]) {
     if (recorder.isRecording) {
       throw ArgumentError(
           '"recorder" must not already be associated with another Canvas.');
     }
     cullRect ??= ui.Rect.largest;
-    _canvas = recorder.beginRecording(cullRect);
+    return SurfaceCanvas._(recorder.beginRecording(cullRect));
   }
 
+  SurfaceCanvas._(this._canvas);
+
   @override
   void save() {
     _canvas.save();
   }
 
   @override
-  void saveLayer(ui.Rect/*?*/ bounds, ui.Paint/*!*/ paint) {
-    assert(paint != null);
+  void saveLayer(ui.Rect? bounds, ui.Paint paint) {
+    assert(paint != null); // ignore: unnecessary_null_comparison
     if (bounds == null) {
       _saveLayerWithoutBounds(paint);
     } else {
@@ -35,11 +35,11 @@ class SurfaceCanvas implements ui.Canvas {
   }
 
   void _saveLayerWithoutBounds(ui.Paint paint) {
-    _canvas.saveLayerWithoutBounds(paint);
+    _canvas.saveLayerWithoutBounds(paint as SurfacePaint);
   }
 
   void _saveLayer(ui.Rect bounds, ui.Paint paint) {
-    _canvas.saveLayer(bounds, paint);
+    _canvas.saveLayer(bounds, paint as SurfacePaint);
   }
 
   @override
@@ -48,33 +48,33 @@ class SurfaceCanvas implements ui.Canvas {
   }
 
   @override
-  int/*!*/ getSaveCount() => _canvas.saveCount;
+  int getSaveCount() => _canvas.saveCount;
 
   @override
-  void translate(double/*!*/ dx, double/*!*/ dy) {
+  void translate(double dx, double dy) {
     _canvas.translate(dx, dy);
   }
 
   @override
-  void scale(double/*!*/ sx, [double/*?*/ sy]) => _scale(sx, sy ?? sx);
+  void scale(double sx, [double? sy]) => _scale(sx, sy ?? sx);
 
   void _scale(double sx, double sy) {
     _canvas.scale(sx, sy);
   }
 
   @override
-  void rotate(double/*!*/ radians) {
+  void rotate(double radians) {
     _canvas.rotate(radians);
   }
 
   @override
-  void skew(double/*!*/ sx, double/*!*/ sy) {
+  void skew(double sx, double sy) {
     _canvas.skew(sx, sy);
   }
 
   @override
-  void transform(Float64List/*!*/ matrix4) {
-    assert(matrix4 != null);
+  void transform(Float64List matrix4) {
+    assert(matrix4 != null); // ignore: unnecessary_null_comparison
     if (matrix4.length != 16) {
       throw ArgumentError('"matrix4" must have 16 entries.');
     }
@@ -86,11 +86,11 @@ class SurfaceCanvas implements ui.Canvas {
   }
 
   @override
-  void clipRect(ui.Rect/*!*/ rect,
-      {ui.ClipOp/*!*/ clipOp = ui.ClipOp.intersect, bool/*!*/ doAntiAlias = true}) {
+  void clipRect(ui.Rect rect,
+      {ui.ClipOp clipOp = ui.ClipOp.intersect, bool doAntiAlias = true}) {
     assert(rectIsValid(rect));
-    assert(clipOp != null);
-    assert(doAntiAlias != null);
+    assert(clipOp != null); // ignore: unnecessary_null_comparison
+    assert(doAntiAlias != null); // ignore: unnecessary_null_comparison
     _clipRect(rect, clipOp, doAntiAlias);
   }
 
@@ -99,9 +99,9 @@ class SurfaceCanvas implements ui.Canvas {
   }
 
   @override
-  void clipRRect(ui.RRect/*!*/ rrect, {bool/*!*/ doAntiAlias = true}) {
+  void clipRRect(ui.RRect rrect, {bool doAntiAlias = true}) {
     assert(rrectIsValid(rrect));
-    assert(doAntiAlias != null);
+    assert(doAntiAlias != null); // ignore: unnecessary_null_comparison
     _clipRRect(rrect, doAntiAlias);
   }
 
@@ -110,9 +110,10 @@ class SurfaceCanvas implements ui.Canvas {
   }
 
   @override
-  void clipPath(ui.Path/*!*/ path, {bool/*!*/ doAntiAlias = true}) {
+  void clipPath(ui.Path path, {bool doAntiAlias = true}) {
+    // ignore: unnecessary_null_comparison
     assert(path != null); // path is checked on the engine side
-    assert(doAntiAlias != null);
+    assert(doAntiAlias != null); // ignore: unnecessary_null_comparison
     _clipPath(path, doAntiAlias);
   }
 
@@ -121,9 +122,9 @@ class SurfaceCanvas implements ui.Canvas {
   }
 
   @override
-  void drawColor(ui.Color/*!*/ color, ui.BlendMode/*!*/ blendMode) {
-    assert(color != null);
-    assert(blendMode != null);
+  void drawColor(ui.Color color, ui.BlendMode blendMode) {
+    assert(color != null); // ignore: unnecessary_null_comparison
+    assert(blendMode != null); // ignore: unnecessary_null_comparison
     _drawColor(color, blendMode);
   }
 
@@ -132,88 +133,88 @@ class SurfaceCanvas implements ui.Canvas {
   }
 
   @override
-  void drawLine(ui.Offset/*!*/ p1, ui.Offset/*!*/ p2, ui.Paint/*!*/ paint) {
+  void drawLine(ui.Offset p1, ui.Offset p2, ui.Paint paint) {
     assert(offsetIsValid(p1));
     assert(offsetIsValid(p2));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawLine(p1, p2, paint);
   }
 
   void _drawLine(ui.Offset p1, ui.Offset p2, ui.Paint paint) {
-    _canvas.drawLine(p1, p2, paint);
+    _canvas.drawLine(p1, p2, paint as SurfacePaint);
   }
 
   @override
-  void drawPaint(ui.Paint/*!*/ paint) {
-    assert(paint != null);
+  void drawPaint(ui.Paint paint) {
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawPaint(paint);
   }
 
   void _drawPaint(ui.Paint paint) {
-    _canvas.drawPaint(paint);
+    _canvas.drawPaint(paint as SurfacePaint);
   }
 
   @override
-  void drawRect(ui.Rect/*!*/ rect, ui.Paint/*!*/ paint) {
+  void drawRect(ui.Rect rect, ui.Paint paint) {
     assert(rectIsValid(rect));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawRect(rect, paint);
   }
 
   void _drawRect(ui.Rect rect, ui.Paint paint) {
-    _canvas.drawRect(rect, paint);
+    _canvas.drawRect(rect, paint as SurfacePaint);
   }
 
   @override
-  void drawRRect(ui.RRect/*!*/ rrect, ui.Paint/*!*/ paint) {
+  void drawRRect(ui.RRect rrect, ui.Paint paint) {
     assert(rrectIsValid(rrect));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawRRect(rrect, paint);
   }
 
   void _drawRRect(ui.RRect rrect, ui.Paint paint) {
-    _canvas.drawRRect(rrect, paint);
+    _canvas.drawRRect(rrect, paint as SurfacePaint);
   }
 
   @override
-  void drawDRRect(ui.RRect/*!*/ outer, ui.RRect/*!*/ inner, ui.Paint/*!*/ paint) {
+  void drawDRRect(ui.RRect outer, ui.RRect inner, ui.Paint paint) {
     assert(rrectIsValid(outer));
     assert(rrectIsValid(inner));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawDRRect(outer, inner, paint);
   }
 
   void _drawDRRect(ui.RRect outer, ui.RRect inner, ui.Paint paint) {
-    _canvas.drawDRRect(outer, inner, paint);
+    _canvas.drawDRRect(outer, inner, paint as SurfacePaint);
   }
 
   @override
-  void drawOval(ui.Rect/*!*/ rect, ui.Paint/*!*/ paint) {
+  void drawOval(ui.Rect rect, ui.Paint paint) {
     assert(rectIsValid(rect));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawOval(rect, paint);
   }
 
   void _drawOval(ui.Rect rect, ui.Paint paint) {
-    _canvas.drawOval(rect, paint);
+    _canvas.drawOval(rect, paint as SurfacePaint);
   }
 
   @override
-  void drawCircle(ui.Offset/*!*/ c, double/*!*/ radius, ui.Paint/*!*/ paint) {
+  void drawCircle(ui.Offset c, double radius, ui.Paint paint) {
     assert(offsetIsValid(c));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawCircle(c, radius, paint);
   }
 
   void _drawCircle(ui.Offset c, double radius, ui.Paint paint) {
-    _canvas.drawCircle(c, radius, paint);
+    _canvas.drawCircle(c, radius, paint as SurfacePaint);
   }
 
   @override
-  void drawArc(ui.Rect/*!*/ rect, double/*!*/ startAngle, double/*!*/ sweepAngle, bool/*!*/ useCenter,
-      ui.Paint/*!*/ paint) {
+  void drawArc(ui.Rect rect, double startAngle, double sweepAngle, bool useCenter,
+      ui.Paint paint) {
     assert(rectIsValid(rect));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     const double pi = math.pi;
     const double pi2 = 2.0 * pi;
 
@@ -243,51 +244,55 @@ class SurfaceCanvas implements ui.Canvas {
     if (useCenter) {
       path.close();
     }
-    _canvas.drawPath(path, paint);
+    _canvas.drawPath(path, paint as SurfacePaint);
   }
 
   @override
-  void drawPath(ui.Path/*!*/ path, ui.Paint/*!*/ paint) {
+  void drawPath(ui.Path path, ui.Paint paint) {
+    // ignore: unnecessary_null_comparison
     assert(path != null); // path is checked on the engine side
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawPath(path, paint);
   }
 
   void _drawPath(ui.Path path, ui.Paint paint) {
-    _canvas.drawPath(path, paint);
+    _canvas.drawPath(path, paint as SurfacePaint);
   }
 
   @override
-  void drawImage(ui.Image/*!*/ image, ui.Offset/*!*/ offset, ui.Paint/*!*/ paint) {
+  void drawImage(ui.Image image, ui.Offset offset, ui.Paint paint) {
+    // ignore: unnecessary_null_comparison
     assert(image != null); // image is checked on the engine side
     assert(offsetIsValid(offset));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawImage(image, offset, paint);
   }
 
   void _drawImage(ui.Image image, ui.Offset p, ui.Paint paint) {
-    _canvas.drawImage(image, p, paint);
+    _canvas.drawImage(image, p, paint as SurfacePaint);
   }
 
   @override
-  void drawImageRect(ui.Image/*!*/ image, ui.Rect/*!*/ src, ui.Rect/*!*/ dst, ui.Paint/*!*/ paint) {
+  void drawImageRect(ui.Image image, ui.Rect src, ui.Rect dst, ui.Paint paint) {
+    // ignore: unnecessary_null_comparison
     assert(image != null); // image is checked on the engine side
     assert(rectIsValid(src));
     assert(rectIsValid(dst));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
     _drawImageRect(image, src, dst, paint);
   }
 
   void _drawImageRect(ui.Image image, ui.Rect src, ui.Rect dst, ui.Paint paint) {
-    _canvas.drawImageRect(image, src, dst, paint);
+    _canvas.drawImageRect(image, src, dst, paint as SurfacePaint);
   }
 
   @override
-  void drawImageNine(ui.Image/*!*/ image, ui.Rect/*!*/ center, ui.Rect/*!*/ dst, ui.Paint/*!*/ paint) {
+  void drawImageNine(ui.Image image, ui.Rect center, ui.Rect dst, ui.Paint paint) {
+    // ignore: unnecessary_null_comparison
     assert(image != null); // image is checked on the engine side
     assert(rectIsValid(center));
     assert(rectIsValid(dst));
-    assert(paint != null);
+    assert(paint != null); // ignore: unnecessary_null_comparison
 
     // Assert you can fit the scaled part of the image (exluding the
     // center source).
@@ -433,15 +438,16 @@ class SurfaceCanvas implements ui.Canvas {
   }
 
   @override
-  void drawPicture(ui.Picture/*!*/ picture) {
+  void drawPicture(ui.Picture picture) {
+    // ignore: unnecessary_null_comparison
     assert(picture != null); // picture is checked on the engine side
     // TODO(het): Support this
     throw UnimplementedError();
   }
 
   @override
-  void drawParagraph(ui.Paragraph/*!*/ paragraph, ui.Offset/*!*/ offset) {
-    assert(paragraph != null);
+  void drawParagraph(ui.Paragraph paragraph, ui.Offset offset) {
+    assert(paragraph != null); // ignore: unnecessary_null_comparison
     assert(offsetIsValid(offset));
     _drawParagraph(paragraph, offset);
   }
@@ -451,52 +457,50 @@ class SurfaceCanvas implements ui.Canvas {
   }
 
   @override
-  void drawPoints(ui.PointMode/*!*/ pointMode, List<ui.Offset/*!*/>/*!*/ points, ui.Paint/*!*/ paint) {
-    assert(pointMode != null);
-    assert(points != null);
-    assert(paint != null);
+  void drawPoints(ui.PointMode pointMode, List<ui.Offset> points, ui.Paint paint) {
+    assert(pointMode != null); // ignore: unnecessary_null_comparison
+    assert(points != null); // ignore: unnecessary_null_comparison
+    assert(paint != null); // ignore: unnecessary_null_comparison
     final Float32List pointList =  offsetListToFloat32List(points);
     drawRawPoints(pointMode, pointList, paint);
   }
 
   @override
-  void drawRawPoints(ui.PointMode/*!*/ pointMode, Float32List/*!*/ points, ui.Paint/*!*/ paint) {
-    assert(pointMode != null);
-    assert(points != null);
-    assert(paint != null);
+  void drawRawPoints(ui.PointMode pointMode, Float32List points, ui.Paint paint) {
+    assert(pointMode != null); // ignore: unnecessary_null_comparison
+    assert(points != null); // ignore: unnecessary_null_comparison
+    assert(paint != null); // ignore: unnecessary_null_comparison
     if (points.length % 2 != 0) {
       throw ArgumentError('"points" must have an even number of values.');
     }
-    _canvas.drawRawPoints(pointMode, points, paint);
+    _canvas.drawRawPoints(pointMode, points, paint as SurfacePaint);
   }
 
   @override
-  void drawVertices(ui.Vertices/*!*/ vertices, ui.BlendMode/*!*/ blendMode, ui.Paint/*!*/ paint) {
-    if (vertices == null) {
-      return;
-    }
+  void drawVertices(ui.Vertices vertices, ui.BlendMode blendMode, ui.Paint paint) {
     //assert(vertices != null); // vertices is checked on the engine side
-    assert(paint != null);
-    assert(blendMode != null);
-    _canvas.drawVertices(vertices, blendMode, paint);
+    assert(paint != null); // ignore: unnecessary_null_comparison
+    assert(blendMode != null); // ignore: unnecessary_null_comparison
+    _canvas.drawVertices(vertices as SurfaceVertices, blendMode, paint as SurfacePaint);
   }
 
   @override
   void drawAtlas(
-    ui.Image/*!*/ atlas,
-    List<ui.RSTransform/*!*/>/*!*/ transforms,
-    List<ui.Rect/*!*/>/*!*/ rects,
-    List<ui.Color/*!*/>/*!*/ colors,
-    ui.BlendMode/*!*/ blendMode,
-    ui.Rect/*?*/ cullRect,
-    ui.Paint/*!*/ paint,
+    ui.Image atlas,
+    List<ui.RSTransform> transforms,
+    List<ui.Rect> rects,
+    List<ui.Color> colors,
+    ui.BlendMode blendMode,
+    ui.Rect? cullRect,
+    ui.Paint paint,
   ) {
+    // ignore: unnecessary_null_comparison
     assert(atlas != null); // atlas is checked on the engine side
-    assert(transforms != null);
-    assert(rects != null);
-    assert(colors != null);
-    assert(blendMode != null);
-    assert(paint != null);
+    assert(transforms != null); // ignore: unnecessary_null_comparison
+    assert(rects != null); // ignore: unnecessary_null_comparison
+    assert(colors != null); // ignore: unnecessary_null_comparison
+    assert(blendMode != null); // ignore: unnecessary_null_comparison
+    assert(paint != null); // ignore: unnecessary_null_comparison
 
     final int rectCount = rects.length;
     if (transforms.length != rectCount) {
@@ -513,20 +517,21 @@ class SurfaceCanvas implements ui.Canvas {
 
   @override
   void drawRawAtlas(
-    ui.Image/*!*/ atlas,
-    Float32List/*!*/ rstTransforms,
-    Float32List/*!*/ rects,
-    Int32List/*!*/ colors,
-    ui.BlendMode/*!*/ blendMode,
-    ui.Rect/*?*/ cullRect,
-    ui.Paint/*!*/ paint,
+    ui.Image atlas,
+    Float32List rstTransforms,
+    Float32List rects,
+    Int32List colors,
+    ui.BlendMode blendMode,
+    ui.Rect? cullRect,
+    ui.Paint paint,
   ) {
+    // ignore: unnecessary_null_comparison
     assert(atlas != null); // atlas is checked on the engine side
-    assert(rstTransforms != null);
-    assert(rects != null);
-    assert(colors != null);
-    assert(blendMode != null);
-    assert(paint != null);
+    assert(rstTransforms != null); // ignore: unnecessary_null_comparison
+    assert(rects != null); // ignore: unnecessary_null_comparison
+    assert(colors != null); // ignore: unnecessary_null_comparison
+    assert(blendMode != null); // ignore: unnecessary_null_comparison
+    assert(paint != null); // ignore: unnecessary_null_comparison
 
     final int rectCount = rects.length;
     if (rstTransforms.length != rectCount) {
@@ -536,7 +541,7 @@ class SurfaceCanvas implements ui.Canvas {
       throw ArgumentError(
           '"rstTransforms" and "rects" lengths must be a multiple of four.');
     }
-    if (colors != null && colors.length * 4 != rectCount) {
+    if (colors.length * 4 != rectCount) {
       throw ArgumentError(
           'If non-null, "colors" length must be one fourth the length of "rstTransforms" and "rects".');
     }
@@ -547,14 +552,15 @@ class SurfaceCanvas implements ui.Canvas {
 
   @override
   void drawShadow(
-    ui.Path/*!*/ path,
-    ui.Color/*!*/ color,
-    double/*!*/ elevation,
-    bool/*!*/ transparentOccluder,
+    ui.Path path,
+    ui.Color color,
+    double elevation,
+    bool transparentOccluder,
   ) {
+    // ignore: unnecessary_null_comparison
     assert(path != null); // path is checked on the engine side
-    assert(color != null);
-    assert(transparentOccluder != null);
+    assert(color != null); // ignore: unnecessary_null_comparison
+    assert(transparentOccluder != null); // ignore: unnecessary_null_comparison
     _canvas.drawShadow(path, color, elevation, transparentOccluder);
   }
 }
diff --git a/lib/web_ui/lib/src/engine/surface/clip.dart b/lib/web_ui/lib/src/engine/surface/clip.dart
index e022195b13f8d..d49be6ab45db4 100644
--- a/lib/web_ui/lib/src/engine/surface/clip.dart
+++ b/lib/web_ui/lib/src/engine/surface/clip.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Mixin used by surfaces that clip their contents using an overflowing DOM
@@ -12,8 +12,8 @@ mixin _DomClip on PersistedContainerSurface {
   /// [rootElement] is used to compensate for the coordinate system shift
   /// introduced by the [rootElement] translation.
   @override
-  html.Element get childContainer => _childContainer;
-  html.Element _childContainer;
+  html.Element? get childContainer => _childContainer;
+  html.Element? _childContainer;
 
   @override
   void adoptElements(_DomClip oldSurface) {
@@ -42,9 +42,9 @@ mixin _DomClip on PersistedContainerSurface {
       // This creates an additional interior element. Count it too.
       _surfaceStatsFor(this).allocatedDomNodeCount++;
     }
-    _childContainer.style.position = 'absolute';
+    _childContainer!.style.position = 'absolute';
 
-    element.append(_childContainer);
+    element.append(_childContainer!);
     return element;
   }
 
@@ -63,13 +63,13 @@ mixin _DomClip on PersistedContainerSurface {
 class PersistedClipRect extends PersistedContainerSurface
     with _DomClip
     implements ui.ClipRectEngineLayer {
-  PersistedClipRect(PersistedClipRect oldLayer, this.rect) : super(oldLayer);
+  PersistedClipRect(PersistedClipRect? oldLayer, this.rect) : super(oldLayer);
 
   final ui.Rect rect;
 
   @override
   void recomputeTransformAndClip() {
-    _transform = parent._transform;
+    _transform = parent!._transform;
     _localClipBounds = rect;
     _localTransformInverse = null;
     _projectedClip = null;
@@ -82,7 +82,7 @@ class PersistedClipRect extends PersistedContainerSurface
 
   @override
   void apply() {
-    rootElement.style
+    rootElement!.style
       ..left = '${rect.left}px'
       ..top = '${rect.top}px'
       ..width = '${rect.right - rect.left}px'
@@ -91,7 +91,7 @@ class PersistedClipRect extends PersistedContainerSurface
     // Translate the child container in the opposite direction to compensate for
     // the shift in the coordinate system introduced by the translation of the
     // rootElement. Clipping in Flutter has no effect on the coordinate system.
-    childContainer.style
+    childContainer!.style
       ..left = '${-rect.left}px'
       ..top = '${-rect.top}px';
   }
@@ -112,16 +112,16 @@ class PersistedClipRect extends PersistedContainerSurface
 class PersistedClipRRect extends PersistedContainerSurface
     with _DomClip
     implements ui.ClipRRectEngineLayer {
-  PersistedClipRRect(ui.EngineLayer oldLayer, this.rrect, this.clipBehavior)
-      : super(oldLayer);
+  PersistedClipRRect(ui.EngineLayer? oldLayer, this.rrect, this.clipBehavior)
+      : super(oldLayer as PersistedSurface?);
 
   final ui.RRect rrect;
   // TODO(yjbanov): can this be controlled in the browser?
-  final ui.Clip clipBehavior;
+  final ui.Clip? clipBehavior;
 
   @override
   void recomputeTransformAndClip() {
-    _transform = parent._transform;
+    _transform = parent!._transform;
     _localClipBounds = rrect.outerRect;
     _localTransformInverse = null;
     _projectedClip = null;
@@ -134,7 +134,7 @@ class PersistedClipRRect extends PersistedContainerSurface
 
   @override
   void apply() {
-    rootElement.style
+    rootElement!.style
       ..left = '${rrect.left}px'
       ..top = '${rrect.top}px'
       ..width = '${rrect.width}px'
@@ -147,7 +147,7 @@ class PersistedClipRRect extends PersistedContainerSurface
     // Translate the child container in the opposite direction to compensate for
     // the shift in the coordinate system introduced by the translation of the
     // rootElement. Clipping in Flutter has no effect on the coordinate system.
-    childContainer.style
+    childContainer!.style
       ..left = '${-rrect.left}px'
       ..top = '${-rrect.top}px';
   }
@@ -167,7 +167,7 @@ class PersistedClipRRect extends PersistedContainerSurface
 class PersistedPhysicalShape extends PersistedContainerSurface
     with _DomClip
     implements ui.PhysicalShapeEngineLayer {
-  PersistedPhysicalShape(PersistedPhysicalShape oldLayer, this.path,
+  PersistedPhysicalShape(PersistedPhysicalShape? oldLayer, this.path,
       this.elevation, int color, int shadowColor, this.clipBehavior)
       : color = ui.Color(color),
         shadowColor = ui.Color(shadowColor),
@@ -180,17 +180,17 @@ class PersistedPhysicalShape extends PersistedContainerSurface
   final ui.Color color;
   final ui.Color shadowColor;
   final ui.Clip clipBehavior;
-  html.Element _clipElement;
+  html.Element? _clipElement;
 
   @override
   void recomputeTransformAndClip() {
-    _transform = parent._transform;
+    _transform = parent!._transform;
 
-    final ui.RRect roundRect = path.webOnlyPathAsRoundedRect;
+    final ui.RRect? roundRect = path.webOnlyPathAsRoundedRect;
     if (roundRect != null) {
       _localClipBounds = roundRect.outerRect;
     } else {
-      final ui.Rect rect = path.webOnlyPathAsRect;
+      final ui.Rect? rect = path.webOnlyPathAsRect;
       if (rect != null) {
         _localClipBounds = rect;
       } else {
@@ -202,7 +202,7 @@ class PersistedPhysicalShape extends PersistedContainerSurface
   }
 
   void _applyColor() {
-    rootElement.style.backgroundColor = colorToCssString(color);
+    rootElement!.style.backgroundColor = colorToCssString(color);
   }
 
   void _applyShadow() {
@@ -222,24 +222,21 @@ class PersistedPhysicalShape extends PersistedContainerSurface
   }
 
   void _applyShape() {
-    if (path == null) {
-      return;
-    }
     // Handle special case of round rect physical shape mapping to
     // rounded div.
-    final ui.RRect roundRect = path.webOnlyPathAsRoundedRect;
+    final ui.RRect? roundRect = path.webOnlyPathAsRoundedRect;
     if (roundRect != null) {
       final String borderRadius =
           '${roundRect.tlRadiusX}px ${roundRect.trRadiusX}px '
           '${roundRect.brRadiusX}px ${roundRect.blRadiusX}px';
-      final html.CssStyleDeclaration style = rootElement.style;
+      final html.CssStyleDeclaration style = rootElement!.style;
       style
         ..left = '${roundRect.left}px'
         ..top = '${roundRect.top}px'
         ..width = '${roundRect.width}px'
         ..height = '${roundRect.height}px'
         ..borderRadius = borderRadius;
-      childContainer.style
+      childContainer!.style
         ..left = '${-roundRect.left}px'
         ..top = '${-roundRect.top}px';
       if (clipBehavior != ui.Clip.none) {
@@ -247,16 +244,16 @@ class PersistedPhysicalShape extends PersistedContainerSurface
       }
       return;
     } else {
-      final ui.Rect rect = path.webOnlyPathAsRect;
+      final ui.Rect? rect = path.webOnlyPathAsRect;
       if (rect != null) {
-        final html.CssStyleDeclaration style = rootElement.style;
+        final html.CssStyleDeclaration style = rootElement!.style;
         style
           ..left = '${rect.left}px'
           ..top = '${rect.top}px'
           ..width = '${rect.width}px'
           ..height = '${rect.height}px'
           ..borderRadius = '';
-        childContainer.style
+        childContainer!.style
           ..left = '${-rect.left}px'
           ..top = '${-rect.top}px';
         if (clipBehavior != ui.Clip.none) {
@@ -264,13 +261,13 @@ class PersistedPhysicalShape extends PersistedContainerSurface
         }
         return;
       } else {
-        final Ellipse ellipse = path.webOnlyPathAsCircle;
+        final Ellipse? ellipse = path.webOnlyPathAsCircle;
         if (ellipse != null) {
           final double rx = ellipse.radiusX;
           final double ry = ellipse.radiusY;
           final String borderRadius =
               rx == ry ? '${rx}px ' : '${rx}px ${ry}px ';
-          final html.CssStyleDeclaration style = rootElement.style;
+          final html.CssStyleDeclaration style = rootElement!.style;
           final double left = ellipse.x - rx;
           final double top = ellipse.y - ry;
           style
@@ -279,7 +276,7 @@ class PersistedPhysicalShape extends PersistedContainerSurface
             ..width = '${rx * 2}px'
             ..height = '${ry * 2}px'
             ..borderRadius = borderRadius;
-          childContainer.style
+          childContainer!.style
             ..left = '${-left}px'
             ..top = '${-top}px';
           if (clipBehavior != ui.Clip.none) {
@@ -298,12 +295,12 @@ class PersistedPhysicalShape extends PersistedContainerSurface
     assert(_clipElement == null);
     _clipElement =
         html.Element.html(svgClipPath, treeSanitizer: _NullTreeSanitizer());
-    domRenderer.append(rootElement, _clipElement);
+    domRenderer.append(rootElement!, _clipElement!);
     domRenderer.setElementStyle(
-        rootElement, 'clip-path', 'url(#svgClip$_clipIdCounter)');
+        rootElement!, 'clip-path', 'url(#svgClip$_clipIdCounter)');
     domRenderer.setElementStyle(
-        rootElement, '-webkit-clip-path', 'url(#svgClip$_clipIdCounter)');
-    final html.CssStyleDeclaration rootElementStyle = rootElement.style;
+        rootElement!, '-webkit-clip-path', 'url(#svgClip$_clipIdCounter)');
+    final html.CssStyleDeclaration rootElementStyle = rootElement!.style;
     rootElementStyle
       ..overflow = ''
       ..left = '${pathBounds.left}px'
@@ -311,7 +308,7 @@ class PersistedPhysicalShape extends PersistedContainerSurface
       ..width = '${pathBounds.width}px'
       ..height = '${pathBounds.height}px'
       ..borderRadius = '';
-    childContainer.style
+    childContainer!.style
       ..left = '-${pathBounds.left}px'
       ..top = '-${pathBounds.top}px';
   }
@@ -328,18 +325,9 @@ class PersistedPhysicalShape extends PersistedContainerSurface
     }
     if (oldSurface.path != path) {
       oldSurface._clipElement?.remove();
-      domRenderer.setElementStyle(rootElement, 'clip-path', '');
-      domRenderer.setElementStyle(rootElement, '-webkit-clip-path', '');
+      domRenderer.setElementStyle(rootElement!, 'clip-path', '');
+      domRenderer.setElementStyle(rootElement!, '-webkit-clip-path', '');
       _applyShape();
-      // This null check is in update since we don't want to unnecessarily
-      // clear style in apply on first build.
-      if (path == null) {
-        // Reset style on prior element when path becomes null.
-        final html.CssStyleDeclaration style = rootElement.style;
-        style.left = '';
-        style.top = '';
-        style.borderRadius = '';
-      }
     } else {
       _clipElement = oldSurface._clipElement;
     }
@@ -351,12 +339,12 @@ class PersistedPhysicalShape extends PersistedContainerSurface
 class PersistedClipPath extends PersistedContainerSurface
     implements ui.ClipPathEngineLayer {
   PersistedClipPath(
-      PersistedClipPath oldLayer, this.clipPath, this.clipBehavior)
+      PersistedClipPath? oldLayer, this.clipPath, this.clipBehavior)
       : super(oldLayer);
 
   final ui.Path clipPath;
   final ui.Clip clipBehavior;
-  html.Element _clipElement;
+  html.Element? _clipElement;
 
   @override
   html.Element createElement() {
@@ -371,20 +359,11 @@ class PersistedClipPath extends PersistedContainerSurface
 
   @override
   void apply() {
-    if (clipPath == null) {
-      if (_clipElement != null) {
-        domRenderer.setElementStyle(childContainer, 'clip-path', '');
-        domRenderer.setElementStyle(childContainer, '-webkit-clip-path', '');
-        _clipElement.remove();
-        _clipElement = null;
-      }
-      return;
-    }
     _clipElement?.remove();
-    final String svgClipPath = createSvgClipDef(childContainer, clipPath);
+    final String svgClipPath = createSvgClipDef(childContainer as html.HtmlElement, clipPath);
     _clipElement =
         html.Element.html(svgClipPath, treeSanitizer: _NullTreeSanitizer());
-    domRenderer.append(childContainer, _clipElement);
+    domRenderer.append(childContainer!, _clipElement!);
   }
 
   @override
diff --git a/lib/web_ui/lib/src/engine/surface/debug_canvas_reuse_overlay.dart b/lib/web_ui/lib/src/engine/surface/debug_canvas_reuse_overlay.dart
index 2b16739da049d..8ac6a13833d3b 100644
--- a/lib/web_ui/lib/src/engine/surface/debug_canvas_reuse_overlay.dart
+++ b/lib/web_ui/lib/src/engine/surface/debug_canvas_reuse_overlay.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 html.HtmlElement _createContainer() {
@@ -32,7 +32,7 @@ class DebugCanvasReuseOverlay {
       ..textDecoration = 'underline';
     title.text = 'Canvas Reuse';
 
-    html.document.body.append(
+    html.document.body!.append(
       container
         ..append(title)
         ..append(
@@ -71,8 +71,8 @@ class DebugCanvasReuseOverlay {
     );
   }
 
-  static DebugCanvasReuseOverlay _instance;
-  static DebugCanvasReuseOverlay/*!*/ get instance {
+  static DebugCanvasReuseOverlay? _instance;
+  static DebugCanvasReuseOverlay get instance {
     if (_instance == null) {
       // Only call the constructor when assertions are enabled to guard against
       // mistakingly including this class in a release build.
@@ -80,7 +80,7 @@ class DebugCanvasReuseOverlay {
         _instance = DebugCanvasReuseOverlay._();
       }
     }
-    return _instance;
+    return _instance!;
   }
 
   final html.Text _created = html.Text('0');
diff --git a/lib/web_ui/lib/src/engine/surface/image_filter.dart b/lib/web_ui/lib/src/engine/surface/image_filter.dart
index 57ee4334a74e6..8ccf727387336 100644
--- a/lib/web_ui/lib/src/engine/surface/image_filter.dart
+++ b/lib/web_ui/lib/src/engine/surface/image_filter.dart
@@ -2,13 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A surface that applies an [imageFilter] to its children.
 class PersistedImageFilter extends PersistedContainerSurface
     implements ui.ImageFilterEngineLayer {
-  PersistedImageFilter(PersistedImageFilter oldLayer, this.filter) : super(oldLayer);
+  PersistedImageFilter(PersistedImageFilter? oldLayer, this.filter) : super(oldLayer);
 
   final ui.ImageFilter filter;
 
@@ -19,7 +19,7 @@ class PersistedImageFilter extends PersistedContainerSurface
 
   @override
   void apply() {
-    rootElement.style.filter = _imageFilterToCss(filter);
+    rootElement!.style.filter = _imageFilterToCss(filter as EngineImageFilter);
   }
 
   @override
diff --git a/lib/web_ui/lib/src/engine/surface/offset.dart b/lib/web_ui/lib/src/engine/surface/offset.dart
index e99623747dd9c..9567edd3da526 100644
--- a/lib/web_ui/lib/src/engine/surface/offset.dart
+++ b/lib/web_ui/lib/src/engine/surface/offset.dart
@@ -2,13 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A surface that translates its children using CSS transform and translate.
 class PersistedOffset extends PersistedContainerSurface
     implements ui.OffsetEngineLayer {
-  PersistedOffset(PersistedOffset oldLayer, this.dx, this.dy) : super(oldLayer);
+  PersistedOffset(PersistedOffset? oldLayer, this.dx, this.dy) : super(oldLayer);
 
   /// Horizontal displacement.
   final double dx;
@@ -18,10 +18,10 @@ class PersistedOffset extends PersistedContainerSurface
 
   @override
   void recomputeTransformAndClip() {
-    _transform = parent._transform;
+    _transform = parent!._transform;
     if (dx != 0.0 || dy != 0.0) {
-      _transform = _transform.clone();
-      _transform.translate(dx, dy);
+      _transform = _transform!.clone();
+      _transform!.translate(dx, dy);
     }
     _projectedClip = null;
     _localTransformInverse = null;
@@ -38,7 +38,7 @@ class PersistedOffset extends PersistedContainerSurface
 
   @override
   void apply() {
-    rootElement.style.transform = 'translate(${dx}px, ${dy}px)';
+    rootElement!.style.transform = 'translate(${dx}px, ${dy}px)';
   }
 
   @override
diff --git a/lib/web_ui/lib/src/engine/surface/opacity.dart b/lib/web_ui/lib/src/engine/surface/opacity.dart
index e867ca8ed6561..fe7b4647dd3a8 100644
--- a/lib/web_ui/lib/src/engine/surface/opacity.dart
+++ b/lib/web_ui/lib/src/engine/surface/opacity.dart
@@ -2,13 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A surface that makes its children transparent.
 class PersistedOpacity extends PersistedContainerSurface
     implements ui.OpacityEngineLayer {
-  PersistedOpacity(PersistedOpacity oldLayer, this.alpha, this.offset)
+  PersistedOpacity(PersistedOpacity? oldLayer, this.alpha, this.offset)
       : super(oldLayer);
 
   final int alpha;
@@ -16,14 +16,14 @@ class PersistedOpacity extends PersistedContainerSurface
 
   @override
   void recomputeTransformAndClip() {
-    _transform = parent._transform;
+    _transform = parent!._transform;
 
     final double dx = offset.dx;
     final double dy = offset.dy;
 
     if (dx != 0.0 || dy != 0.0) {
-      _transform = _transform.clone();
-      _transform.translate(dx, dy);
+      _transform = _transform!.clone();
+      _transform!.translate(dx, dy);
     }
     _localTransformInverse = null;
     _projectedClip = null;
@@ -43,8 +43,8 @@ class PersistedOpacity extends PersistedContainerSurface
     // TODO(yjbanov): evaluate using `filter: opacity(X)`. It is a longer string
     //                but it reportedly has better hardware acceleration, so may
     //                be worth the trade-off.
-    rootElement.style.opacity = '${alpha / 255}';
-    rootElement.style.transform = 'translate(${offset.dx}px, ${offset.dy}px)';
+    rootElement!.style.opacity = '${alpha / 255}';
+    rootElement!.style.transform = 'translate(${offset.dx}px, ${offset.dy}px)';
   }
 
   @override
diff --git a/lib/web_ui/lib/src/engine/surface/painting.dart b/lib/web_ui/lib/src/engine/surface/painting.dart
index cce4564ea6782..f98687c32ec89 100644
--- a/lib/web_ui/lib/src/engine/surface/painting.dart
+++ b/lib/web_ui/lib/src/engine/surface/painting.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Implementation of [ui.Paint] used by the HTML rendering backend.
@@ -46,7 +46,7 @@ class SurfacePaint implements ui.Paint {
   }
 
   @override
-  ui.StrokeCap get strokeCap => _paintData.strokeCap;
+  ui.StrokeCap get strokeCap => _paintData.strokeCap ?? ui.StrokeCap.butt;
 
   @override
   set strokeCap(ui.StrokeCap value) {
@@ -58,7 +58,7 @@ class SurfacePaint implements ui.Paint {
   }
 
   @override
-  ui.StrokeJoin get strokeJoin => _paintData.strokeJoin;
+  ui.StrokeJoin get strokeJoin => _paintData.strokeJoin ?? ui.StrokeJoin.miter;
 
   @override
   set strokeJoin(ui.StrokeJoin value) {
@@ -82,7 +82,7 @@ class SurfacePaint implements ui.Paint {
   }
 
   @override
-  ui.Color get color => _paintData.color;
+  ui.Color get color => _paintData.color ?? _defaultPaintColor;
 
   @override
   set color(ui.Color value) {
@@ -105,10 +105,10 @@ class SurfacePaint implements ui.Paint {
   static const ui.Color _defaultPaintColor = ui.Color(0xFF000000);
 
   @override
-  ui.Shader get shader => _paintData.shader;
+  ui.Shader? get shader => _paintData.shader;
 
   @override
-  set shader(ui.Shader value) {
+  set shader(ui.Shader? value) {
     if (_frozen) {
       _paintData = _paintData.clone();
       _frozen = false;
@@ -117,10 +117,10 @@ class SurfacePaint implements ui.Paint {
   }
 
   @override
-  ui.MaskFilter get maskFilter => _paintData.maskFilter;
+  ui.MaskFilter? get maskFilter => _paintData.maskFilter;
 
   @override
-  set maskFilter(ui.MaskFilter value) {
+  set maskFilter(ui.MaskFilter? value) {
     if (_frozen) {
       _paintData = _paintData.clone();
       _frozen = false;
@@ -129,7 +129,7 @@ class SurfacePaint implements ui.Paint {
   }
 
   @override
-  ui.FilterQuality get filterQuality => _paintData.filterQuality;
+  ui.FilterQuality get filterQuality => _paintData.filterQuality ?? ui.FilterQuality.none;
 
   @override
   set filterQuality(ui.FilterQuality value) {
@@ -141,10 +141,10 @@ class SurfacePaint implements ui.Paint {
   }
 
   @override
-  ui.ColorFilter get colorFilter => _paintData.colorFilter;
+  ui.ColorFilter? get colorFilter => _paintData.colorFilter;
 
   @override
-  set colorFilter(ui.ColorFilter value) {
+  set colorFilter(ui.ColorFilter? value) {
     if (_frozen) {
       _paintData = _paintData.clone();
       _frozen = false;
@@ -155,22 +155,22 @@ class SurfacePaint implements ui.Paint {
   // TODO(flutter_web): see https://github.com/flutter/flutter/issues/33605
   @override
   double get strokeMiterLimit {
-    return null;
+    throw UnsupportedError('SurfacePaint.strokeMiterLimit');
   }
 
   @override
   set strokeMiterLimit(double value) {
-    assert(value != null);
+    assert(value != null); // ignore: unnecessary_null_comparison
   }
 
   @override
-  ui.ImageFilter get imageFilter {
+  ui.ImageFilter? get imageFilter {
     // TODO(flutter/flutter#35156): Implement ImageFilter.
     return null;
   }
 
   @override
-  set imageFilter(ui.ImageFilter value) {
+  set imageFilter(ui.ImageFilter? value) {
     // TODO(flutter/flutter#35156): Implement ImageFilter.
   }
 
@@ -192,11 +192,11 @@ class SurfacePaint implements ui.Paint {
     result.write('Paint(');
     if (style == ui.PaintingStyle.stroke) {
       result.write('$style');
-      if (strokeWidth != null && strokeWidth != 0.0)
+      if (strokeWidth != 0.0)
         result.write(' $strokeWidth');
       else
         result.write(' hairline');
-      if (strokeCap != null && strokeCap != ui.StrokeCap.butt)
+      if (strokeCap != ui.StrokeCap.butt)
         result.write(' $strokeCap');
       semicolon = '; ';
     }
@@ -205,10 +205,7 @@ class SurfacePaint implements ui.Paint {
       semicolon = '; ';
     }
     if (color != _defaultPaintColor) {
-      if (color != null)
-        result.write('$semicolon$color');
-      else
-        result.write('${semicolon}no color');
+      result.write('$semicolon$color');
       semicolon = '; ';
     }
     result.write(')');
@@ -219,17 +216,17 @@ class SurfacePaint implements ui.Paint {
 /// Private Paint context data used for recording canvas commands allowing
 /// Paint to be mutated post canvas draw operations.
 class SurfacePaintData {
-  ui.BlendMode blendMode;
-  ui.PaintingStyle style;
-  double strokeWidth;
-  ui.StrokeCap strokeCap;
-  ui.StrokeJoin strokeJoin;
+  ui.BlendMode? blendMode;
+  ui.PaintingStyle? style;
+  double? strokeWidth;
+  ui.StrokeCap? strokeCap;
+  ui.StrokeJoin? strokeJoin;
   bool isAntiAlias = true;
-  ui.Color color;
-  ui.Shader shader;
-  ui.MaskFilter maskFilter;
-  ui.FilterQuality filterQuality;
-  ui.ColorFilter colorFilter;
+  ui.Color? color;
+  ui.Shader? shader;
+  ui.MaskFilter? maskFilter;
+  ui.FilterQuality? filterQuality;
+  ui.ColorFilter? colorFilter;
 
   // Internal for recording canvas use.
   SurfacePaintData clone() {
diff --git a/lib/web_ui/lib/src/engine/surface/path.dart b/lib/web_ui/lib/src/engine/surface/path.dart
index de9b90a24d1f1..7d08c284219ee 100644
--- a/lib/web_ui/lib/src/engine/surface/path.dart
+++ b/lib/web_ui/lib/src/engine/surface/path.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A complex, one-dimensional subset of a plane.
@@ -26,18 +26,18 @@ class SurfacePath implements ui.Path {
   final List<Subpath> subpaths;
   ui.PathFillType _fillType = ui.PathFillType.nonZero;
 
-  Subpath get _currentSubpath => subpaths.isEmpty ? null : subpaths.last;
+  Subpath get _currentSubpath => subpaths.last;
 
-  List<PathCommand> get _commands => _currentSubpath?.commands;
+  List<PathCommand> get _commands => _currentSubpath.commands;
 
   /// The current x-coordinate for this path.
-  double get _currentX => _currentSubpath?.currentX ?? 0.0;
+  double get _currentX => subpaths.isNotEmpty ? _currentSubpath.currentX : 0.0;
 
   /// The current y-coordinate for this path.
-  double get _currentY => _currentSubpath?.currentY ?? 0.0;
+  double get _currentY => subpaths.isNotEmpty ? _currentSubpath.currentY : 0.0;
 
   /// Recorder used for hit testing paths.
-  static RawRecordingCanvas _rawRecorder;
+  static RawRecordingCanvas? _rawRecorder;
 
   SurfacePath() : subpaths = <Subpath>[];
 
@@ -465,7 +465,7 @@ class SurfacePath implements ui.Path {
   /// The `points` argument is interpreted as offsets from the origin.
   @override
   void addPolygon(List<ui.Offset> points, bool close) {
-    assert(points != null);
+    assert(points != null); // ignore: unnecessary_null_comparison
     if (points.isEmpty) {
       return;
     }
@@ -504,13 +504,14 @@ class SurfacePath implements ui.Path {
   /// after the matrix is translated by the given offset. The matrix is a 4x4
   /// matrix stored in column major order.
   @override
-  void addPath(ui.Path path, ui.Offset offset, {Float64List matrix4}) {
+  void addPath(ui.Path path, ui.Offset offset, {Float64List? matrix4}) {
+    // ignore: unnecessary_null_comparison
     assert(path != null); // path is checked on the engine side
     assert(offsetIsValid(offset));
     if (matrix4 != null) {
-      _addPathWithMatrix(path, offset.dx, offset.dy, toMatrix32(matrix4));
+      _addPathWithMatrix(path as SurfacePath, offset.dx, offset.dy, toMatrix32(matrix4));
     } else {
-      _addPath(path, offset.dx, offset.dy);
+      _addPath(path as SurfacePath, offset.dx, offset.dy);
     }
   }
 
@@ -539,15 +540,16 @@ class SurfacePath implements ui.Path {
   /// after the matrix is translated by the given `offset`.  The matrix is a 4x4
   /// matrix stored in column major order.
   @override
-  void extendWithPath(ui.Path path, ui.Offset offset, {Float64List matrix4}) {
+  void extendWithPath(ui.Path path, ui.Offset offset, {Float64List? matrix4}) {
+    // ignore: unnecessary_null_comparison
     assert(path != null); // path is checked on the engine side
     assert(offsetIsValid(offset));
     if (matrix4 != null) {
       final Float32List matrix32 = toMatrix32(matrix4);
       assert(matrix4IsValid(matrix32));
-      _extendWithPathAndMatrix(path, offset.dx, offset.dy, matrix32);
+      _extendWithPathAndMatrix(path as SurfacePath, offset.dx, offset.dy, matrix32);
     } else {
-      _extendWithPath(path, offset.dx, offset.dy);
+      _extendWithPath(path as SurfacePath, offset.dx, offset.dy);
     }
   }
 
@@ -668,22 +670,22 @@ class SurfacePath implements ui.Path {
     final ui.Size size = window.physicalSize;
     // If device pixel ratio has changed we can't reuse prior raw recorder.
     if (_rawRecorder != null &&
-        _rawRecorder._devicePixelRatio !=
+        _rawRecorder!._devicePixelRatio !=
             EngineWindow.browserDevicePixelRatio) {
       _rawRecorder = null;
     }
     final double dpr = window.devicePixelRatio;
-    _rawRecorder ??=
+    final RawRecordingCanvas rawRecorder = _rawRecorder ??=
         RawRecordingCanvas(ui.Size(size.width / dpr, size.height / dpr));
     // Account for the shift due to padding.
-    _rawRecorder.translate(-BitmapCanvas.kPaddingPixels.toDouble(),
+    rawRecorder.translate(-BitmapCanvas.kPaddingPixels.toDouble(),
         -BitmapCanvas.kPaddingPixels.toDouble());
-    _rawRecorder.drawPath(
+    rawRecorder.drawPath(
         this, (SurfacePaint()..color = const ui.Color(0xFF000000)).paintData);
-    final double recorderDevicePixelRatio = _rawRecorder._devicePixelRatio;
-    final bool result = _rawRecorder._canvasPool.context.isPointInPath(
+    final double recorderDevicePixelRatio = rawRecorder._devicePixelRatio;
+    final bool result = rawRecorder._canvasPool.context.isPointInPath(
         pointX * recorderDevicePixelRatio, pointY * recorderDevicePixelRatio);
-    _rawRecorder.dispose();
+    rawRecorder.dispose();
     return result;
   }
 
@@ -746,17 +748,17 @@ class SurfacePath implements ui.Path {
         bool skipBounds = false;
         switch (op.type) {
           case PathCommandTypes.moveTo:
-            final MoveTo cmd = op;
+            final MoveTo cmd = op as MoveTo;
             curX = minX = maxX = cmd.x;
             curY = minY = maxY = cmd.y;
             break;
           case PathCommandTypes.lineTo:
-            final LineTo cmd = op;
+            final LineTo cmd = op as LineTo;
             curX = minX = maxX = cmd.x;
             curY = minY = maxY = cmd.y;
             break;
           case PathCommandTypes.ellipse:
-            final Ellipse cmd = op;
+            final Ellipse cmd = op as Ellipse;
             // Rotate 4 corners of bounding box.
             final double rx = cmd.radiusX;
             final double ry = cmd.radiusY;
@@ -805,7 +807,7 @@ class SurfacePath implements ui.Path {
             curY = centerY;
             break;
           case PathCommandTypes.quadraticCurveTo:
-            final QuadraticCurveTo cmd = op;
+            final QuadraticCurveTo cmd = op as QuadraticCurveTo;
             final double x1 = curX;
             final double y1 = curY;
             final double cpX = cmd.x1;
@@ -827,14 +829,14 @@ class SurfacePath implements ui.Path {
 
             double denom = x1 - (2 * cpX) + x2;
             if (denom.abs() > epsilon) {
-              final num t1 = (x1 - cpX) / denom;
+              final double t1 = (x1 - cpX) / denom;
               if ((t1 >= 0) && (t1 <= 1.0)) {
                 // Solve (x,y) for curve at t = tx to find extrema
-                final num tprime = 1.0 - t1;
-                final num extremaX = (tprime * tprime * x1) +
+                final double tprime = 1.0 - t1;
+                final double extremaX = (tprime * tprime * x1) +
                     (2 * t1 * tprime * cpX) +
                     (t1 * t1 * x2);
-                final num extremaY = (tprime * tprime * y1) +
+                final double extremaY = (tprime * tprime * y1) +
                     (2 * t1 * tprime * cpY) +
                     (t1 * t1 * y2);
                 // Expand bounds.
@@ -847,13 +849,13 @@ class SurfacePath implements ui.Path {
             // Now calculate dy/dt = 0
             denom = y1 - (2 * cpY) + y2;
             if (denom.abs() > epsilon) {
-              final num t2 = (y1 - cpY) / denom;
+              final double t2 = (y1 - cpY) / denom;
               if ((t2 >= 0) && (t2 <= 1.0)) {
-                final num tprime2 = 1.0 - t2;
-                final num extrema2X = (tprime2 * tprime2 * x1) +
+                final double tprime2 = 1.0 - t2;
+                final double extrema2X = (tprime2 * tprime2 * x1) +
                     (2 * t2 * tprime2 * cpX) +
                     (t2 * t2 * x2);
-                final num extrema2Y = (tprime2 * tprime2 * y1) +
+                final double extrema2Y = (tprime2 * tprime2 * y1) +
                     (2 * t2 * tprime2 * cpY) +
                     (t2 * t2 * y2);
                 // Expand bounds.
@@ -867,7 +869,7 @@ class SurfacePath implements ui.Path {
             curY = y2;
             break;
           case PathCommandTypes.bezierCurveTo:
-            final BezierCurveTo cmd = op;
+            final BezierCurveTo cmd = op as BezierCurveTo;
             final double startX = curX;
             final double startY = curY;
             final double cpX1 = cmd.x1;
@@ -902,13 +904,13 @@ class SurfacePath implements ui.Path {
               // Now find roots for quadratic equation with known coefficients
               // a,b,c
               // The roots are (-b+-sqrt(b*b-4*a*c)) / 2a
-              num s = (b * b) - (4 * a * c);
+              double s = (b * b) - (4 * a * c);
               // If s is negative, we have no real roots
               if ((s >= 0.0) && (a.abs() > epsilon)) {
                 if (s == 0.0) {
                   // we have only 1 root
-                  final num t = -b / (2 * a);
-                  final num tprime = 1.0 - t;
+                  final double t = -b / (2 * a);
+                  final double tprime = 1.0 - t;
                   if ((t >= 0.0) && (t <= 1.0)) {
                     extremaX = ((tprime * tprime * tprime) * startX) +
                         ((3 * tprime * tprime * t) * cpX1) +
@@ -920,8 +922,8 @@ class SurfacePath implements ui.Path {
                 } else {
                   // we have 2 roots
                   s = math.sqrt(s);
-                  num t = (-b - s) / (2 * a);
-                  num tprime = 1.0 - t;
+                  double t = (-b - s) / (2 * a);
+                  double tprime = 1.0 - t;
                   if ((t >= 0.0) && (t <= 1.0)) {
                     extremaX = ((tprime * tprime * tprime) * startX) +
                         ((3 * tprime * tprime * t) * cpX1) +
@@ -960,13 +962,13 @@ class SurfacePath implements ui.Path {
               // Now find roots for quadratic equation with known coefficients
               // a,b,c
               // The roots are (-b+-sqrt(b*b-4*a*c)) / 2a
-              num s = (b * b) - (4 * a * c);
+              double s = (b * b) - (4 * a * c);
               // If s is negative, we have no real roots
               if ((s >= 0.0) && (a.abs() > epsilon)) {
                 if (s == 0.0) {
                   // we have only 1 root
-                  final num t = -b / (2 * a);
-                  final num tprime = 1.0 - t;
+                  final double t = -b / (2 * a);
+                  final double tprime = 1.0 - t;
                   if ((t >= 0.0) && (t <= 1.0)) {
                     extremaY = ((tprime * tprime * tprime) * startY) +
                         ((3 * tprime * tprime * t) * cpY1) +
@@ -978,8 +980,8 @@ class SurfacePath implements ui.Path {
                 } else {
                   // we have 2 roots
                   s = math.sqrt(s);
-                  final num t = (-b - s) / (2 * a);
-                  final num tprime = 1.0 - t;
+                  final double t = (-b - s) / (2 * a);
+                  final double tprime = 1.0 - t;
                   if ((t >= 0.0) && (t <= 1.0)) {
                     extremaY = ((tprime * tprime * tprime) * startY) +
                         ((3 * tprime * tprime * t) * cpY1) +
@@ -989,8 +991,8 @@ class SurfacePath implements ui.Path {
                     maxY = math.max(extremaY, maxY);
                   }
                   // check 2nd root
-                  final num t2 = (-b + s) / (2 * a);
-                  final num tprime2 = 1.0 - t2;
+                  final double t2 = (-b + s) / (2 * a);
+                  final double tprime2 = 1.0 - t2;
                   if ((t2 >= 0.0) && (t2 <= 1.0)) {
                     extremaY = ((tprime2 * tprime2 * tprime2) * startY) +
                         ((3 * tprime2 * tprime2 * t2) * cpY1) +
@@ -1006,7 +1008,7 @@ class SurfacePath implements ui.Path {
             curY = endY;
             break;
           case PathCommandTypes.rect:
-            final RectCommand cmd = op;
+            final RectCommand cmd = op as RectCommand;
             minX = cmd.x;
             double width = cmd.width;
             if (cmd.width < 0) {
@@ -1025,7 +1027,7 @@ class SurfacePath implements ui.Path {
             maxY = minY + height;
             break;
           case PathCommandTypes.rRect:
-            final RRectCommand cmd = op;
+            final RRectCommand cmd = op as RRectCommand;
             final ui.RRect rRect = cmd.rrect;
             curX = minX = rRect.left;
             maxX = rRect.left + rRect.width;
@@ -1072,7 +1074,7 @@ class SurfacePath implements ui.Path {
   ///
   /// Used for web optimization of physical shape represented as
   /// a persistent div.
-  ui.RRect get webOnlyPathAsRoundedRect {
+  ui.RRect? get webOnlyPathAsRoundedRect {
     if (subpaths.length != 1) {
       return null;
     }
@@ -1088,7 +1090,7 @@ class SurfacePath implements ui.Path {
   ///
   /// Used for web optimization of physical shape represented as
   /// a persistent div.
-  ui.Rect get webOnlyPathAsRect {
+  ui.Rect? get webOnlyPathAsRect {
     if (subpaths.length != 1) {
       return null;
     }
@@ -1106,7 +1108,7 @@ class SurfacePath implements ui.Path {
   ///
   /// Used for web optimization of physical shape represented as
   /// a persistent div.
-  Ellipse get webOnlyPathAsCircle {
+  Ellipse? get webOnlyPathAsCircle {
     if (subpaths.length != 1) {
       return null;
     }
diff --git a/lib/web_ui/lib/src/engine/surface/path_metrics.dart b/lib/web_ui/lib/src/engine/surface/path_metrics.dart
index 7ad30fd725dc3..7ac19658256ae 100644
--- a/lib/web_ui/lib/src/engine/surface/path_metrics.dart
+++ b/lib/web_ui/lib/src/engine/surface/path_metrics.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
 const double kEpsilon = 0.000000001;
@@ -36,10 +35,7 @@ class SurfacePathMetrics extends IterableBase<ui.PathMetric>
 /// Maintains a single instance of computed segments for set of PathMetric
 /// objects exposed through iterator.
 class _SurfacePathMeasure {
-  _SurfacePathMeasure(this._path, this.forceClosed) {
-    // nextContour will increment this to the zero based index.
-    _currentContourIndex = -1;
-  }
+  _SurfacePathMeasure(this._path, this.forceClosed);
 
   final SurfacePath _path;
   final List<_PathContourMeasure> _contours = [];
@@ -48,12 +44,12 @@ class _SurfacePathMeasure {
   // have been implied when using [Path.addRect])
   final bool forceClosed;
 
-  int _currentContourIndex;
+  // nextContour will increment this to the zero based index.
+  int _currentContourIndex = -1;
   int get currentContourIndex => _currentContourIndex;
 
   // Iterator index into [Path.subPaths]
   int _subPathIndex = -1;
-  _PathContourMeasure _contourMeasure;
 
   double length(int contourIndex) {
     assert(contourIndex <= currentContourIndex,
@@ -71,7 +67,7 @@ class _SurfacePathMeasure {
   /// Returns null if the contour has zero [length].
   ///
   /// The distance is clamped to the [length] of the current contour.
-  ui.Tangent getTangentForOffset(int contourIndex, double distance) {
+  ui.Tangent? getTangentForOffset(int contourIndex, double distance) {
     return _contours[contourIndex].getTangentForOffset(distance);
   }
 
@@ -104,13 +100,11 @@ class _SurfacePathMeasure {
       return false;
     }
     ++_subPathIndex;
-    _contourMeasure =
-        _PathContourMeasure(_path.subpaths[_subPathIndex], forceClosed);
-    _contours.add(_contourMeasure);
+    _contours.add(_PathContourMeasure(_path.subpaths[_subPathIndex], forceClosed));
     return true;
   }
 
-  ui.Path extractPath(int contourIndex, double start, double end,
+  ui.Path? extractPath(int contourIndex, double start, double end,
       {bool startWithMoveTo = true}) {
     return _contours[contourIndex].extractPath(start, end, startWithMoveTo);
   }
@@ -133,10 +127,10 @@ class _PathContourMeasure {
   double get length => _contourLength;
   bool get isClosed => _isClosed;
 
-  double _contourLength = 0.0;
+  double _contourLength = 0;
   bool _isClosed = false;
 
-  ui.Tangent getTangentForOffset(double distance) {
+  ui.Tangent? getTangentForOffset(double distance) {
     final segmentIndex = _segmentIndexAtDistance(distance);
     if (segmentIndex == -1) {
       return null;
@@ -190,7 +184,7 @@ class _PathContourMeasure {
     return segment.computeTangent(t);
   }
 
-  ui.Path extractPath(
+  ui.Path? extractPath(
       double startDistance, double stopDistance, bool startWithMoveTo) {
     if (startDistance < 0) {
       startDistance = 0;
@@ -284,23 +278,25 @@ class _PathContourMeasure {
       currentX = x;
       currentY = y;
     };
-    _EllipseSegmentResult ellipseResult;
+
+    // TODO(yjbanov): make late final (https://github.com/dart-lang/sdk/issues/42422)
+    _EllipseSegmentResult? ellipseResult;
     for (PathCommand command in commands) {
       switch (command.type) {
         case PathCommandTypes.moveTo:
-          final MoveTo moveTo = command;
+          final MoveTo moveTo = command as MoveTo;
           currentX = moveTo.x;
           currentY = moveTo.y;
           haveSeenMoveTo = true;
           break;
         case PathCommandTypes.lineTo:
           assert(haveSeenMoveTo);
-          final LineTo lineTo = command;
+          final LineTo lineTo = command as LineTo;
           lineToHandler(lineTo.x, lineTo.y);
           break;
         case PathCommandTypes.bezierCurveTo:
           assert(haveSeenMoveTo);
-          final BezierCurveTo curve = command;
+          final BezierCurveTo curve = command as BezierCurveTo;
           // Compute cubic curve distance.
           distance = _computeCubicSegments(
               currentX,
@@ -318,7 +314,7 @@ class _PathContourMeasure {
           break;
         case PathCommandTypes.quadraticCurveTo:
           assert(haveSeenMoveTo);
-          final QuadraticCurveTo quadraticCurveTo = command;
+          final QuadraticCurveTo quadraticCurveTo = command as QuadraticCurveTo;
           // Compute quad curve distance.
           distance = _computeQuadSegments(
               currentX,
@@ -334,7 +330,7 @@ class _PathContourMeasure {
         case PathCommandTypes.close:
           break;
         case PathCommandTypes.ellipse:
-          final Ellipse ellipse = command;
+          final Ellipse ellipse = command as Ellipse;
           ellipseResult ??= _EllipseSegmentResult();
           _computeEllipseSegments(
               currentX,
@@ -348,15 +344,15 @@ class _PathContourMeasure {
               ellipse.radiusX,
               ellipse.radiusY,
               ellipse.anticlockwise,
-              ellipseResult,
+              ellipseResult!,
               _segments);
-          distance = ellipseResult.distance;
-          currentX = ellipseResult.endPointX;
-          currentY = ellipseResult.endPointY;
+          distance = ellipseResult!.distance;
+          currentX = ellipseResult!.endPointX;
+          currentY = ellipseResult!.endPointY;
           _isClosed = true;
           break;
         case PathCommandTypes.rRect:
-          final RRectCommand rrectCommand = command;
+          final RRectCommand rrectCommand = command as RRectCommand;
           final ui.RRect rrect = rrectCommand.rrect;
           RRectMetricsRenderer(moveToCallback: (double x, double y) {
             currentX = x;
@@ -386,16 +382,16 @@ class _PathContourMeasure {
                 radiusX,
                 radiusY,
                 antiClockwise,
-                ellipseResult,
+                ellipseResult!,
                 _segments);
-            distance = ellipseResult.distance;
-            currentX = ellipseResult.endPointX;
-            currentY = ellipseResult.endPointY;
+            distance = ellipseResult!.distance;
+            currentX = ellipseResult!.endPointX;
+            currentY = ellipseResult!.endPointY;
           }).render(rrect);
           _isClosed = true;
           break;
         case PathCommandTypes.rect:
-          final RectCommand rectCommand = command;
+          final RectCommand rectCommand = command as RectCommand;
           final double x = rectCommand.x;
           final double y = rectCommand.y;
           final double width = rectCommand.width;
@@ -612,13 +608,14 @@ class _PathContourMeasure {
 
 /// Tracks iteration from one segment of a path to the next for measurement.
 class SurfacePathMetricIterator implements Iterator<ui.PathMetric> {
+  // ignore: unnecessary_null_comparison
   SurfacePathMetricIterator._(this._pathMeasure) : assert(_pathMeasure != null);
 
-  SurfacePathMetric _pathMetric;
+  SurfacePathMetric? _pathMetric;
   _SurfacePathMeasure _pathMeasure;
 
   @override
-  SurfacePathMetric get current => _pathMetric;
+  SurfacePathMetric get current => _pathMetric!;
 
   @override
   bool moveNext() {
@@ -655,7 +652,7 @@ const double _fTolerance = 0.5;
 /// to maintain consistency with native platforms.
 class SurfacePathMetric implements ui.PathMetric {
   SurfacePathMetric._(this._measure)
-      : assert(_measure != null),
+      : assert(_measure != null), // ignore: unnecessary_null_comparison
         length = _measure.length(_measure.currentContourIndex),
         isClosed = _measure.isClosed(_measure.currentContourIndex),
         contourIndex = _measure.currentContourIndex;
@@ -699,7 +696,7 @@ class SurfacePathMetric implements ui.PathMetric {
   ///
   /// The distance is clamped to the [length] of the current contour.
   @override
-  ui.Tangent getTangentForOffset(double distance) {
+  ui.Tangent? getTangentForOffset(double distance) {
     return _measure.getTangentForOffset(contourIndex, distance);
   }
 
@@ -708,7 +705,7 @@ class SurfacePathMetric implements ui.PathMetric {
   /// `start` and `end` are pinned to legal values (0..[length])
   /// Returns null if the segment is 0 length or `start` > `stop`.
   /// Begin the segment with a moveTo if `startWithMoveTo` is true.
-  ui.Path extractPath(double start, double end, {bool startWithMoveTo = true}) {
+  ui.Path? extractPath(double start, double end, {bool startWithMoveTo = true}) {
     return _measure.extractPath(contourIndex, start, end,
         startWithMoveTo: startWithMoveTo);
   }
@@ -718,10 +715,9 @@ class SurfacePathMetric implements ui.PathMetric {
 }
 
 class _EllipseSegmentResult {
-  double endPointX;
-  double endPointY;
-  double distance;
-  _EllipseSegmentResult();
+  double endPointX = 0;
+  double endPointY = 0;
+  double distance = 0;
 }
 
 // Given a vector dx, dy representing slope, normalize and return as [ui.Offset].
@@ -734,9 +730,9 @@ ui.Offset _normalizeSlope(double dx, double dy) {
 
 class _SurfaceTangent extends ui.Tangent {
   const _SurfaceTangent(ui.Offset position, ui.Offset vector, this.t)
-      : assert(position != null),
-        assert(vector != null),
-        assert(t != null),
+      : assert(position != null), // ignore: unnecessary_null_comparison
+        assert(vector != null), // ignore: unnecessary_null_comparison
+        assert(t != null), // ignore: unnecessary_null_comparison
         super(position, vector);
 
   // Normalized distance of tangent point from start of a contour.
diff --git a/lib/web_ui/lib/src/engine/surface/picture.dart b/lib/web_ui/lib/src/engine/surface/picture.dart
index cb67a0e3e526d..2e57ff1279972 100644
--- a/lib/web_ui/lib/src/engine/surface/picture.dart
+++ b/lib/web_ui/lib/src/engine/surface/picture.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 // TODO(yjbanov): this is currently very naive. We probably want to cache
@@ -36,10 +36,10 @@ void _reduceCanvasMemoryUsage() {
 /// canvases forcing us to allocate new large canvases.
 class _PaintRequest {
   _PaintRequest({
-    this.canvasSize,
-    this.paintCallback,
-  })  : assert(canvasSize != null),
-        assert(paintCallback != null);
+    required this.canvasSize,
+    required this.paintCallback,
+  })  : assert(canvasSize != null), // ignore: unnecessary_null_comparison
+        assert(paintCallback != null); // ignore: unnecessary_null_comparison
 
   final ui.Size canvasSize;
   final ui.VoidCallback paintCallback;
@@ -50,7 +50,7 @@ class _PaintRequest {
 /// the number of reusable canvases.
 List<_PaintRequest> _paintQueue = <_PaintRequest>[];
 
-void _recycleCanvas(EngineCanvas canvas) {
+void _recycleCanvas(EngineCanvas? canvas) {
   if (canvas is BitmapCanvas) {
     if (canvas.isReusable()) {
       _recycledCanvases.add(canvas);
@@ -99,7 +99,7 @@ PersistedHoudiniPicture houdiniPictureFactory(
 
 class PersistedHoudiniPicture extends PersistedPicture {
   PersistedHoudiniPicture(double dx, double dy, ui.Picture picture, int hints)
-      : super(dx, dy, picture, hints) {
+      : super(dx, dy, picture as EnginePicture, hints) {
     if (!_cssPainterRegistered) {
       _registerCssPainter();
     }
@@ -139,20 +139,20 @@ class PersistedHoudiniPicture extends PersistedPicture {
   int get bitmapPixelCount => 0;
 
   @override
-  void applyPaint(EngineCanvas oldCanvas) {
+  void applyPaint(EngineCanvas? oldCanvas) {
     _recycleCanvas(oldCanvas);
     final HoudiniCanvas canvas = HoudiniCanvas(_optimalLocalCullRect);
     _canvas = canvas;
-    domRenderer.clearDom(rootElement);
-    rootElement.append(_canvas.rootElement);
-    picture.recordingCanvas.apply(_canvas, _optimalLocalCullRect);
+    domRenderer.clearDom(rootElement!);
+    rootElement!.append(_canvas!.rootElement);
+    picture.recordingCanvas!.apply(_canvas, _optimalLocalCullRect);
     canvas.commit();
   }
 }
 
 class PersistedStandardPicture extends PersistedPicture {
   PersistedStandardPicture(double dx, double dy, ui.Picture picture, int hints)
-      : super(dx, dy, picture, hints);
+      : super(dx, dy, picture as EnginePicture, hints);
 
   @override
   double matchForUpdate(PersistedStandardPicture existingSurface) {
@@ -161,15 +161,15 @@ class PersistedStandardPicture extends PersistedPicture {
       return 0.0;
     }
 
-    if (!existingSurface.picture.recordingCanvas.didDraw) {
+    if (!existingSurface.picture.recordingCanvas!.didDraw) {
       // The previous surface didn't draw anything and therefore has no
       // resources to reuse.
       return 1.0;
     }
 
     final bool didRequireBitmap =
-        existingSurface.picture.recordingCanvas.hasArbitraryPaint;
-    final bool requiresBitmap = picture.recordingCanvas.hasArbitraryPaint;
+        existingSurface.picture.recordingCanvas!.hasArbitraryPaint;
+    final bool requiresBitmap = picture.recordingCanvas!.hasArbitraryPaint;
     if (didRequireBitmap != requiresBitmap) {
       // Switching canvas types is always expensive.
       return 1.0;
@@ -179,17 +179,17 @@ class PersistedStandardPicture extends PersistedPicture {
       // which point we may return other values here.
       return 1.0;
     } else {
-      final BitmapCanvas oldCanvas = existingSurface._canvas;
+      final BitmapCanvas? oldCanvas = existingSurface._canvas as BitmapCanvas?;
       if (oldCanvas == null) {
         // We did not allocate a canvas last time. This can happen when the
         // picture is completely clipped out of the view.
         return 1.0;
-      } else if (!oldCanvas.doesFitBounds(_exactLocalCullRect)) {
+      } else if (!oldCanvas.doesFitBounds(_exactLocalCullRect!)) {
         // The canvas needs to be resized before painting.
         return 1.0;
       } else {
-        final int newPixelCount = BitmapCanvas._widthToPhysical(_exactLocalCullRect.width)
-             * BitmapCanvas._heightToPhysical(_exactLocalCullRect.height);
+        final int newPixelCount = BitmapCanvas._widthToPhysical(_exactLocalCullRect!.width)
+             * BitmapCanvas._heightToPhysical(_exactLocalCullRect!.height);
         final int oldPixelCount =
             oldCanvas._widthInBitmapPixels * oldCanvas._heightInBitmapPixels;
 
@@ -206,7 +206,7 @@ class PersistedStandardPicture extends PersistedPicture {
   }
 
   @override
-  Matrix4 get localTransformInverse => null;
+  Matrix4? get localTransformInverse => null;
 
   @override
   int get bitmapPixelCount {
@@ -214,39 +214,39 @@ class PersistedStandardPicture extends PersistedPicture {
       return 0;
     }
 
-    final BitmapCanvas bitmapCanvas = _canvas;
+    final BitmapCanvas bitmapCanvas = _canvas as BitmapCanvas;
     return bitmapCanvas.bitmapPixelCount;
   }
 
   @override
-  void applyPaint(EngineCanvas oldCanvas) {
-    if (picture.recordingCanvas.hasArbitraryPaint) {
+  void applyPaint(EngineCanvas? oldCanvas) {
+    if (picture.recordingCanvas!.hasArbitraryPaint) {
       _applyBitmapPaint(oldCanvas);
     } else {
       _applyDomPaint(oldCanvas);
     }
   }
 
-  void _applyDomPaint(EngineCanvas oldCanvas) {
+  void _applyDomPaint(EngineCanvas? oldCanvas) {
     _recycleCanvas(oldCanvas);
     _canvas = DomCanvas();
-    domRenderer.clearDom(rootElement);
-    rootElement.append(_canvas.rootElement);
-    picture.recordingCanvas.apply(_canvas, _optimalLocalCullRect);
+    domRenderer.clearDom(rootElement!);
+    rootElement!.append(_canvas!.rootElement);
+    picture.recordingCanvas!.apply(_canvas, _optimalLocalCullRect);
   }
 
-  void _applyBitmapPaint(EngineCanvas oldCanvas) {
+  void _applyBitmapPaint(EngineCanvas? oldCanvas) {
     if (oldCanvas is BitmapCanvas &&
-        oldCanvas.doesFitBounds(_optimalLocalCullRect) &&
+        oldCanvas.doesFitBounds(_optimalLocalCullRect!) &&
         oldCanvas.isReusable()) {
       if (_debugShowCanvasReuseStats) {
         DebugCanvasReuseOverlay.instance.keptCount++;
       }
-      oldCanvas.bounds = _optimalLocalCullRect;
+      oldCanvas.bounds = _optimalLocalCullRect!;
       _canvas = oldCanvas;
       oldCanvas.setElementCache(_elementCache);
-      _canvas.clear();
-      picture.recordingCanvas.apply(_canvas, _optimalLocalCullRect);
+      _canvas!.clear();
+      picture.recordingCanvas!.apply(_canvas, _optimalLocalCullRect);
     } else {
       // We can't use the old canvas because the size has changed, so we put
       // it in a cache for later reuse.
@@ -256,20 +256,20 @@ class PersistedStandardPicture extends PersistedPicture {
       // picture to be painted after the update cycle is done syncing the layer
       // tree then reuse canvases that were freed up.
       _paintQueue.add(_PaintRequest(
-        canvasSize: _optimalLocalCullRect.size,
+        canvasSize: _optimalLocalCullRect!.size,
         paintCallback: () {
-          _canvas = _findOrCreateCanvas(_optimalLocalCullRect);
+          _canvas = _findOrCreateCanvas(_optimalLocalCullRect!);
           assert(_canvas is BitmapCanvas
-              && (_canvas as BitmapCanvas)._elementCache == _elementCache);
+              && (_canvas as BitmapCanvas?)!._elementCache == _elementCache);
           if (_debugExplainSurfaceStats) {
-            final BitmapCanvas bitmapCanvas = _canvas;
+            final BitmapCanvas bitmapCanvas = _canvas as BitmapCanvas;
             _surfaceStatsFor(this).paintPixelCount +=
                 bitmapCanvas.bitmapPixelCount;
           }
-          domRenderer.clearDom(rootElement);
-          rootElement.append(_canvas.rootElement);
-          _canvas.clear();
-          picture.recordingCanvas.apply(_canvas, _optimalLocalCullRect);
+          domRenderer.clearDom(rootElement!);
+          rootElement!.append(_canvas!.rootElement);
+          _canvas!.clear();
+          picture.recordingCanvas!.apply(_canvas, _optimalLocalCullRect);
         },
       ));
     }
@@ -288,7 +288,7 @@ class PersistedStandardPicture extends PersistedPicture {
   ///   sure we do not use too much memory for small canvases.
   BitmapCanvas _findOrCreateCanvas(ui.Rect bounds) {
     final ui.Size canvasSize = bounds.size;
-    BitmapCanvas bestRecycledCanvas;
+    BitmapCanvas? bestRecycledCanvas;
     double lastPixelCount = double.infinity;
     for (int i = 0; i < _recycledCanvases.length; i++) {
       final BitmapCanvas candidate = _recycledCanvases[i];
@@ -358,19 +358,19 @@ class PersistedStandardPicture extends PersistedPicture {
 /// to draw shapes and text.
 abstract class PersistedPicture extends PersistedLeafSurface {
   PersistedPicture(this.dx, this.dy, this.picture, this.hints)
-      : localPaintBounds = picture.recordingCanvas.pictureBounds;
+      : localPaintBounds = picture.recordingCanvas!.pictureBounds;
 
-  EngineCanvas _canvas;
+  EngineCanvas? _canvas;
 
   /// Returns the canvas used by this picture layer.
   ///
   /// Useful for tests.
-  EngineCanvas get debugCanvas => _canvas;
+  EngineCanvas? get debugCanvas => _canvas;
 
   final double dx;
   final double dy;
   final EnginePicture picture;
-  final ui.Rect localPaintBounds;
+  final ui.Rect? localPaintBounds;
   final int hints;
 
   /// Cache for reusing elements such as images across picture updates.
@@ -384,10 +384,10 @@ abstract class PersistedPicture extends PersistedLeafSurface {
 
   @override
   void recomputeTransformAndClip() {
-    _transform = parent._transform;
+    _transform = parent!._transform;
     if (dx != 0.0 || dy != 0.0) {
-      _transform = _transform.clone();
-      _transform.translate(dx, dy);
+      _transform = _transform!.clone();
+      _transform!.translate(dx, dy);
     }
     _computeExactCullRects();
   }
@@ -399,14 +399,14 @@ abstract class PersistedPicture extends PersistedLeafSurface {
   /// contain everything that's visible, but it may be bigger than necessary.
   /// Therefore it should not be used for clipping. It is meant to be used for
   /// optimizing canvas allocation.
-  ui.Rect get optimalLocalCullRect => _optimalLocalCullRect;
-  ui.Rect _optimalLocalCullRect;
+  ui.Rect? get optimalLocalCullRect => _optimalLocalCullRect;
+  ui.Rect? _optimalLocalCullRect;
 
   /// Same as [optimalLocalCullRect] but in screen coordinate system.
-  ui.Rect get debugExactGlobalCullRect => _exactGlobalCullRect;
-  ui.Rect _exactGlobalCullRect;
+  ui.Rect? get debugExactGlobalCullRect => _exactGlobalCullRect;
+  ui.Rect? _exactGlobalCullRect;
 
-  ui.Rect _exactLocalCullRect;
+  ui.Rect? _exactLocalCullRect;
 
   /// Computes the canvas paint bounds based on the estimated paint bounds and
   /// the scaling produced by transformations.
@@ -420,15 +420,15 @@ abstract class PersistedPicture extends PersistedLeafSurface {
     assert(transform != null);
     assert(localPaintBounds != null);
 
-    if (parent._projectedClip == null) {
+    if (parent!._projectedClip == null) {
       // Compute and cache chain of clipping bounds on parent of picture since
       // parent may include multiple pictures so it can be reused by all
       // child pictures.
-      ui.Rect bounds;
-      PersistedSurface parentSurface = parent;
+      ui.Rect? bounds;
+      PersistedSurface? parentSurface = parent;
       final Matrix4 clipTransform = Matrix4.identity();
       while (parentSurface != null) {
-        final ui.Rect localClipBounds = parentSurface._localClipBounds;
+        final ui.Rect? localClipBounds = parentSurface._localClipBounds;
         if (localClipBounds != null) {
           if (bounds == null) {
             bounds = transformRect(clipTransform, localClipBounds);
@@ -437,7 +437,7 @@ abstract class PersistedPicture extends PersistedLeafSurface {
                 bounds.intersect(transformRect(clipTransform, localClipBounds));
           }
         }
-        final Matrix4 localInverse = parentSurface.localTransformInverse;
+        final Matrix4? localInverse = parentSurface.localTransformInverse;
         if (localInverse != null && !localInverse.isIdentity()) {
           clipTransform.multiply(localInverse);
         }
@@ -447,30 +447,30 @@ abstract class PersistedPicture extends PersistedLeafSurface {
         bounds = ui.Rect.zero;
       }
       // Cache projected clip on parent.
-      parent._projectedClip = bounds;
+      parent!._projectedClip = bounds;
     }
     // Intersect localPaintBounds with parent projected clip to calculate
     // and cache [_exactLocalCullRect].
-    if (parent._projectedClip == null) {
+    if (parent!._projectedClip == null) {
       _exactLocalCullRect = localPaintBounds;
     } else {
-      _exactLocalCullRect = localPaintBounds.intersect(parent._projectedClip);
+      _exactLocalCullRect = localPaintBounds!.intersect(parent!._projectedClip!);
     }
-    if (_exactLocalCullRect.width <= 0 || _exactLocalCullRect.height <= 0) {
+    if (_exactLocalCullRect!.width <= 0 || _exactLocalCullRect!.height <= 0) {
       _exactLocalCullRect = ui.Rect.zero;
       _exactGlobalCullRect = ui.Rect.zero;
     } else {
       assert(() {
-        _exactGlobalCullRect = transformRect(transform, _exactLocalCullRect);
+        _exactGlobalCullRect = transformRect(transform!, _exactLocalCullRect!);
         return true;
       }());
     }
   }
 
-  bool _computeOptimalCullRect(PersistedPicture oldSurface) {
+  bool _computeOptimalCullRect(PersistedPicture? oldSurface) {
     assert(_exactLocalCullRect != null);
 
-    if (oldSurface == null || !oldSurface.picture.recordingCanvas.didDraw) {
+    if (oldSurface == null || !oldSurface.picture.recordingCanvas!.didDraw) {
       // First useful paint.
       _optimalLocalCullRect = _exactLocalCullRect;
       return true;
@@ -479,7 +479,7 @@ abstract class PersistedPicture extends PersistedLeafSurface {
     assert(oldSurface._optimalLocalCullRect != null);
 
     final bool surfaceBeingRetained = identical(oldSurface, this);
-    final ui.Rect oldOptimalLocalCullRect = surfaceBeingRetained
+    final ui.Rect? oldOptimalLocalCullRect = surfaceBeingRetained
         ? _optimalLocalCullRect
         : oldSurface._optimalLocalCullRect;
 
@@ -490,7 +490,7 @@ abstract class PersistedPicture extends PersistedLeafSurface {
       return oldOptimalLocalCullRect != ui.Rect.zero;
     }
 
-    if (rectContainsOther(oldOptimalLocalCullRect, _exactLocalCullRect)) {
+    if (rectContainsOther(oldOptimalLocalCullRect!, _exactLocalCullRect!)) {
       // The cull rect we computed in the past contains the newly computed cull
       // rect. This can happen, for example, when the picture is being shrunk by
       // a clip when it is scrolled out of the screen. In this case we do not
@@ -512,18 +512,18 @@ abstract class PersistedPicture extends PersistedLeafSurface {
 
     // Compute the delta, by which each of the side of the clip rect has "moved"
     // since the last time we updated the cull rect.
-    final double leftwardDelta = oldOptimalLocalCullRect.left - _exactLocalCullRect.left;
-    final double upwardDelta = oldOptimalLocalCullRect.top - _exactLocalCullRect.top;
-    final double rightwardDelta = _exactLocalCullRect.right - oldOptimalLocalCullRect.right;
-    final double bottomwardDelta = _exactLocalCullRect.bottom - oldOptimalLocalCullRect.bottom;
+    final double leftwardDelta = oldOptimalLocalCullRect.left - _exactLocalCullRect!.left;
+    final double upwardDelta = oldOptimalLocalCullRect.top - _exactLocalCullRect!.top;
+    final double rightwardDelta = _exactLocalCullRect!.right - oldOptimalLocalCullRect.right;
+    final double bottomwardDelta = _exactLocalCullRect!.bottom - oldOptimalLocalCullRect.bottom;
 
     // Compute the new optimal rect to paint into.
     final ui.Rect newLocalCullRect = ui.Rect.fromLTRB(
-      _exactLocalCullRect.left - _predictTrend(leftwardDelta, _exactLocalCullRect.width),
-      _exactLocalCullRect.top - _predictTrend(upwardDelta, _exactLocalCullRect.height),
-      _exactLocalCullRect.right + _predictTrend(rightwardDelta, _exactLocalCullRect.width),
-      _exactLocalCullRect.bottom + _predictTrend(bottomwardDelta, _exactLocalCullRect.height),
-    ).intersect(localPaintBounds);
+      _exactLocalCullRect!.left - _predictTrend(leftwardDelta, _exactLocalCullRect!.width),
+      _exactLocalCullRect!.top - _predictTrend(upwardDelta, _exactLocalCullRect!.height),
+      _exactLocalCullRect!.right + _predictTrend(rightwardDelta, _exactLocalCullRect!.width),
+      _exactLocalCullRect!.bottom + _predictTrend(bottomwardDelta, _exactLocalCullRect!.height),
+    ).intersect(localPaintBounds!);
 
     final bool localCullRectChanged = _optimalLocalCullRect != newLocalCullRect;
     _optimalLocalCullRect = newLocalCullRect;
@@ -555,14 +555,14 @@ abstract class PersistedPicture extends PersistedLeafSurface {
   /// return zero.
   int get bitmapPixelCount;
 
-  void _applyPaint(PersistedPicture oldSurface) {
-    final EngineCanvas oldCanvas = oldSurface?._canvas;
-    if (!picture.recordingCanvas.didDraw || _optimalLocalCullRect.isEmpty) {
+  void _applyPaint(PersistedPicture? oldSurface) {
+    final EngineCanvas? oldCanvas = oldSurface?._canvas;
+    if (!picture.recordingCanvas!.didDraw || _optimalLocalCullRect!.isEmpty) {
       // The picture is empty, or it has been completely clipped out. Skip
       // painting. This removes all the setup work and scaffolding objects
       // that won't be useful for anything anyway.
       _recycleCanvas(oldCanvas);
-      domRenderer.clearDom(rootElement);
+      domRenderer.clearDom(rootElement!);
       _canvas = null;
       return;
     }
@@ -576,10 +576,10 @@ abstract class PersistedPicture extends PersistedLeafSurface {
   }
 
   /// Concrete implementations implement this method to do actual painting.
-  void applyPaint(EngineCanvas oldCanvas);
+  void applyPaint(EngineCanvas? oldCanvas);
 
   void _applyTranslate() {
-    rootElement.style.transform = 'translate(${dx}px, ${dy}px)';
+    rootElement!.style.transform = 'translate(${dx}px, ${dy}px)';
   }
 
   @override
@@ -640,14 +640,14 @@ abstract class PersistedPicture extends PersistedLeafSurface {
   @override
   void debugPrintChildren(StringBuffer buffer, int indent) {
     super.debugPrintChildren(buffer, indent);
-    if (rootElement != null && rootElement.firstChild != null) {
-      final html.Element firstChild = rootElement.firstChild;
+    if (rootElement != null && rootElement!.firstChild != null) {
+      final html.Element firstChild = rootElement!.firstChild as html.Element;
       final String canvasTag = firstChild.tagName.toLowerCase();
-      final int canvasHash = rootElement.firstChild.hashCode;
+      final int canvasHash = rootElement!.firstChild!.hashCode;
       buffer.writeln('${'  ' * (indent + 1)}<$canvasTag @$canvasHash />');
     } else if (rootElement != null) {
       buffer.writeln(
-          '${'  ' * (indent + 1)}<${rootElement.tagName.toLowerCase()} @$hashCode />');
+          '${'  ' * (indent + 1)}<${rootElement!.tagName.toLowerCase()} @$hashCode />');
     } else {
       buffer.writeln('${'  ' * (indent + 1)}<recycled-canvas />');
     }
@@ -657,8 +657,8 @@ abstract class PersistedPicture extends PersistedLeafSurface {
   void debugValidate(List<String> validationErrors) {
     super.debugValidate(validationErrors);
 
-    if (picture.recordingCanvas.didDraw) {
-      if (!_optimalLocalCullRect.isEmpty && debugCanvas == null) {
+    if (picture.recordingCanvas!.didDraw) {
+      if (!_optimalLocalCullRect!.isEmpty && debugCanvas == null) {
         validationErrors
             .add('$runtimeType has non-trivial picture but it has null canvas');
       }
diff --git a/lib/web_ui/lib/src/engine/surface/platform_view.dart b/lib/web_ui/lib/src/engine/surface/platform_view.dart
index 8f0fd5cfd0de2..df75492cc8b82 100644
--- a/lib/web_ui/lib/src/engine/surface/platform_view.dart
+++ b/lib/web_ui/lib/src/engine/surface/platform_view.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// A surface containing a platform view, which is an HTML element.
@@ -13,7 +13,7 @@ class PersistedPlatformView extends PersistedLeafSurface {
   final double width;
   final double height;
 
-  html.ShadowRoot _shadowRoot;
+  late html.ShadowRoot _shadowRoot;
 
   PersistedPlatformView(this.viewId, this.dx, this.dy, this.width, this.height);
 
@@ -46,7 +46,7 @@ class PersistedPlatformView extends PersistedLeafSurface {
         all: initial;
       }''';
     _shadowRoot.append(_styleReset);
-    final html.Element platformView =
+    final html.Element? platformView =
         ui.platformViewRegistry.getCreatedView(viewId);
     if (platformView != null) {
       _shadowRoot.append(platformView);
@@ -57,16 +57,16 @@ class PersistedPlatformView extends PersistedLeafSurface {
   }
 
   @override
-  Matrix4 get localTransformInverse => null;
+  Matrix4? get localTransformInverse => null;
 
   @override
   void apply() {
-    rootElement.style
+    rootElement!.style
       ..transform = 'translate(${dx}px, ${dy}px)'
       ..width = '${width}px'
       ..height = '${height}px';
     // Set size of the root element created by the PlatformView.
-    final html.Element platformView =
+    final html.Element? platformView =
         ui.platformViewRegistry.getCreatedView(viewId);
     if (platformView != null) {
       platformView.style
diff --git a/lib/web_ui/lib/src/engine/surface/recording_canvas.dart b/lib/web_ui/lib/src/engine/surface/recording_canvas.dart
index 650b2d5e4e41b..7ae1c8edcb2ea 100644
--- a/lib/web_ui/lib/src/engine/surface/recording_canvas.dart
+++ b/lib/web_ui/lib/src/engine/surface/recording_canvas.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Enable this to print every command applied by a canvas.
@@ -34,14 +34,14 @@ class RawRecordingCanvas extends BitmapCanvas
   @override
   ui.Picture endRecording() => throw UnsupportedError('');
 
-  RecordingCanvas _canvas; // ignore: unused_field
+  RecordingCanvas? _canvas; // ignore: unused_field
 
   bool _isRecording = true; // ignore: unused_field
 
   @override
   bool get isRecording => true;
 
-  ui.Rect cullRect;
+  ui.Rect? cullRect;
 }
 
 /// Records canvas commands to be applied to a [EngineCanvas].
@@ -56,14 +56,14 @@ class RecordingCanvas {
   /// The bounds contain the full picture. The commands recorded for the picture
   /// are later pruned based on the clip applied to the picture. See the [apply]
   /// method for more details.
-  ui.Rect get pictureBounds {
+  ui.Rect? get pictureBounds {
     assert(
       _debugRecordingEnded,
       'Picture bounds not available yet. Call [endRecording] before accessing picture bounds.',
     );
     return _pictureBounds;
   }
-  ui.Rect _pictureBounds;
+  ui.Rect? _pictureBounds;
 
   final List<PaintCommand> _commands = <PaintCommand>[];
 
@@ -75,7 +75,7 @@ class RecordingCanvas {
     throw UnsupportedError('For debugging only.');
   }
 
-  RecordingCanvas(ui.Rect bounds) : _paintBounds = _PaintBounds(bounds);
+  RecordingCanvas(ui.Rect? bounds) : _paintBounds = _PaintBounds(bounds ?? ui.Rect.largest);
 
   /// Whether this canvas is doing arbitrary paint operations not expressible
   /// via DOM elements.
@@ -125,14 +125,14 @@ class RecordingCanvas {
   /// a minimum). The commands that fall outside the clip are skipped and are
   /// not applied to the [engineCanvas]. A command must have a non-zero
   /// intersection with the clip in order to be applied.
-  void apply(EngineCanvas engineCanvas, ui.Rect clipRect) {
+  void apply(EngineCanvas? engineCanvas, ui.Rect? clipRect) {
     assert(_debugRecordingEnded);
     if (_debugDumpPaintCommands) {
       final StringBuffer debugBuf = StringBuffer();
       int skips = 0;
       debugBuf.writeln(
-          '--- Applying RecordingCanvas to ${engineCanvas.runtimeType} '
-          'with bounds $_paintBounds and clip $clipRect (w = ${clipRect.width},'
+          '--- Applying RecordingCanvas to ${engineCanvas!.runtimeType} '
+          'with bounds $_paintBounds and clip $clipRect (w = ${clipRect!.width},'
           ' h = ${clipRect.height})');
       for (int i = 0; i < _commands.length; i++) {
         final PaintCommand command = _commands[i];
@@ -154,7 +154,7 @@ class RecordingCanvas {
       print(debugBuf);
     } else {
       try {
-        if (rectContainsOther(clipRect, _pictureBounds)) {
+        if (rectContainsOther(clipRect!, _pictureBounds!)) {
           // No need to check if commands fit in the clip rect if we already
           // know that the entire picture fits it.
           for (int i = 0, len = _commands.length; i < len; i++) {
@@ -182,11 +182,11 @@ class RecordingCanvas {
         }
       }
     }
-    engineCanvas.endOfPaint();
+    engineCanvas!.endOfPaint();
   }
 
   /// Prints recorded commands.
-  String debugPrintCommands() {
+  String? debugPrintCommands() {
     if (assertionsEnabled) {
       final StringBuffer debugBuf = StringBuffer();
       for (int i = 0; i < _commands.length; i++) {
@@ -287,7 +287,7 @@ class RecordingCanvas {
 
   void clipPath(ui.Path path, {bool doAntiAlias = true}) {
     assert(!_debugRecordingEnded);
-    final PaintClipPath command = PaintClipPath(path);
+    final PaintClipPath command = PaintClipPath(path as SurfacePath);
     _paintBounds.clipRect(path.getBounds(), command);
     _hasArbitraryPaint = true;
     _commands.add(command);
@@ -453,19 +453,19 @@ class RecordingCanvas {
     if (paint.shader == null) {
       // For Rect/RoundedRect paths use drawRect/drawRRect code paths for
       // DomCanvas optimization.
-      SurfacePath sPath = path;
-      final ui.Rect rect = sPath.webOnlyPathAsRect;
+      SurfacePath sPath = path as SurfacePath;
+      final ui.Rect? rect = sPath.webOnlyPathAsRect;
       if (rect != null) {
         drawRect(rect, paint);
         return;
       }
-      final ui.RRect rrect = sPath.webOnlyPathAsRoundedRect;
+      final ui.RRect? rrect = sPath.webOnlyPathAsRoundedRect;
       if (rrect != null) {
         drawRRect(rrect, paint);
         return;
       }
     }
-    SurfacePath sPath = path;
+    SurfacePath sPath = path as SurfacePath;
     if (sPath.subpaths.isNotEmpty) {
       _hasArbitraryPaint = true;
       _didDraw = true;
@@ -476,7 +476,7 @@ class RecordingCanvas {
       }
       // Clone path so it can be reused for subsequent draw calls.
       final ui.Path clone = SurfacePath._shallowCopy(path);
-      final PaintDrawPath command = PaintDrawPath(clone, paint.paintData);
+      final PaintDrawPath command = PaintDrawPath(clone as SurfacePath, paint.paintData);
       _paintBounds.grow(pathBounds, command);
       clone.fillType = sPath.fillType;
       _commands.add(command);
@@ -506,7 +506,7 @@ class RecordingCanvas {
 
   void drawParagraph(ui.Paragraph paragraph, ui.Offset offset) {
     assert(!_debugRecordingEnded);
-    final EngineParagraph engineParagraph = paragraph;
+    final EngineParagraph engineParagraph = paragraph as EngineParagraph;
     if (!engineParagraph._isLaidOut) {
       // Ignore non-laid out paragraphs. This matches Flutter's behavior.
       return;
@@ -536,7 +536,7 @@ class RecordingCanvas {
     _didDraw = true;
     final ui.Rect shadowRect =
         computePenumbraBounds(path.getBounds(), elevation);
-    final PaintDrawShadow command = PaintDrawShadow(path, color, elevation, transparentOccluder);
+    final PaintDrawShadow command = PaintDrawShadow(path as SurfacePath, color, elevation, transparentOccluder);
     _paintBounds.grow(shadowRect, command);
     _commands.add(command);
   }
@@ -554,9 +554,6 @@ class RecordingCanvas {
   void drawRawPoints(
       ui.PointMode pointMode, Float32List points, SurfacePaint paint) {
     assert(!_debugRecordingEnded);
-    if (paint.strokeWidth == null) {
-      return;
-    }
     _hasArbitraryPaint = true;
     _didDraw = true;
     final PaintDrawPoints command = PaintDrawPoints(pointMode, points, paint.paintData);
@@ -606,7 +603,7 @@ class RecordingCanvas {
 abstract class PaintCommand {
   const PaintCommand();
 
-  void apply(EngineCanvas canvas);
+  void apply(EngineCanvas? canvas);
 
   void serializeToCssPaint(List<List<dynamic>> serializedCommands);
 }
@@ -634,14 +631,14 @@ abstract class DrawCommand extends PaintCommand {
   double bottomBound = double.infinity;
 
   /// Whether this command intersects with the [clipRect].
-  bool isInvisible(ui.Rect clipRect) {
+  bool isInvisible(ui.Rect? clipRect) {
     if (isClippedOut) {
       return true;
     }
 
     // Check top and bottom first because vertical scrolling is more common
     // than horizontal scrolling.
-    return bottomBound < clipRect.top ||
+    return bottomBound < clipRect!.top ||
       topBound > clipRect.bottom ||
       rightBound < clipRect.left ||
       leftBound > clipRect.right;
@@ -652,8 +649,8 @@ class PaintSave extends PaintCommand {
   const PaintSave();
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.save();
+  void apply(EngineCanvas? canvas) {
+    canvas!.save();
   }
 
   @override
@@ -675,8 +672,8 @@ class PaintRestore extends PaintCommand {
   const PaintRestore();
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.restore();
+  void apply(EngineCanvas? canvas) {
+    canvas!.restore();
   }
 
   @override
@@ -701,8 +698,8 @@ class PaintTranslate extends PaintCommand {
   PaintTranslate(this.dx, this.dy);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.translate(dx, dy);
+  void apply(EngineCanvas? canvas) {
+    canvas!.translate(dx, dy);
   }
 
   @override
@@ -727,8 +724,8 @@ class PaintScale extends PaintCommand {
   PaintScale(this.sx, this.sy);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.scale(sx, sy);
+  void apply(EngineCanvas? canvas) {
+    canvas!.scale(sx, sy);
   }
 
   @override
@@ -752,8 +749,8 @@ class PaintRotate extends PaintCommand {
   PaintRotate(this.radians);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.rotate(radians);
+  void apply(EngineCanvas? canvas) {
+    canvas!.rotate(radians);
   }
 
   @override
@@ -777,8 +774,8 @@ class PaintTransform extends PaintCommand {
   PaintTransform(this.matrix4);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.transform(matrix4);
+  void apply(EngineCanvas? canvas) {
+    canvas!.transform(matrix4);
   }
 
   @override
@@ -803,8 +800,8 @@ class PaintSkew extends PaintCommand {
   PaintSkew(this.sx, this.sy);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.skew(sx, sy);
+  void apply(EngineCanvas? canvas) {
+    canvas!.skew(sx, sy);
   }
 
   @override
@@ -828,8 +825,8 @@ class PaintClipRect extends DrawCommand {
   PaintClipRect(this.rect);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.clipRect(rect);
+  void apply(EngineCanvas? canvas) {
+    canvas!.clipRect(rect);
   }
 
   @override
@@ -853,8 +850,8 @@ class PaintClipRRect extends DrawCommand {
   PaintClipRRect(this.rrect);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.clipRRect(rrect);
+  void apply(EngineCanvas? canvas) {
+    canvas!.clipRRect(rrect);
   }
 
   @override
@@ -881,8 +878,8 @@ class PaintClipPath extends DrawCommand {
   PaintClipPath(this.path);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.clipPath(path);
+  void apply(EngineCanvas? canvas) {
+    canvas!.clipPath(path);
   }
 
   @override
@@ -907,8 +904,8 @@ class PaintDrawColor extends DrawCommand {
   PaintDrawColor(this.color, this.blendMode);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.drawColor(color, blendMode);
+  void apply(EngineCanvas? canvas) {
+    canvas!.drawColor(color, blendMode);
   }
 
   @override
@@ -935,8 +932,8 @@ class PaintDrawLine extends DrawCommand {
   PaintDrawLine(this.p1, this.p2, this.paint);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.drawLine(p1, p2, paint);
+  void apply(EngineCanvas? canvas) {
+    canvas!.drawLine(p1, p2, paint);
   }
 
   @override
@@ -967,8 +964,8 @@ class PaintDrawPaint extends DrawCommand {
   PaintDrawPaint(this.paint);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.drawPaint(paint);
+  void apply(EngineCanvas? canvas) {
+    canvas!.drawPaint(paint);
   }
 
   @override
@@ -993,8 +990,8 @@ class PaintDrawVertices extends DrawCommand {
   PaintDrawVertices(this.vertices, this.blendMode, this.paint);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.drawVertices(vertices, blendMode, paint);
+  void apply(EngineCanvas? canvas) {
+    canvas!.drawVertices(vertices as SurfaceVertices, blendMode, paint);
   }
 
   @override
@@ -1019,8 +1016,8 @@ class PaintDrawPoints extends DrawCommand {
   PaintDrawPoints(this.pointMode, this.points, this.paint);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.drawPoints(pointMode, points, paint);
+  void apply(EngineCanvas? canvas) {
+    canvas!.drawPoints(pointMode, points, paint);
   }
 
   @override
@@ -1045,8 +1042,8 @@ class PaintDrawRect extends DrawCommand {
   PaintDrawRect(this.rect, this.paint);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.drawRect(rect, paint);
+  void apply(EngineCanvas? canvas) {
+    canvas!.drawRect(rect, paint);
   }
 
   @override
@@ -1075,8 +1072,8 @@ class PaintDrawRRect extends DrawCommand {
   PaintDrawRRect(this.rrect, this.paint);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.drawRRect(rrect, paint);
+  void apply(EngineCanvas? canvas) {
+    canvas!.drawRRect(rrect, paint);
   }
 
   @override
@@ -1106,8 +1103,8 @@ class PaintDrawDRRect extends DrawCommand {
   PaintDrawDRRect(this.outer, this.inner, this.paint);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.drawDRRect(outer, inner, paint);
+  void apply(EngineCanvas? canvas) {
+    canvas!.drawDRRect(outer, inner, paint);
   }
 
   @override
@@ -1137,8 +1134,8 @@ class PaintDrawOval extends DrawCommand {
   PaintDrawOval(this.rect, this.paint);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.drawOval(rect, paint);
+  void apply(EngineCanvas? canvas) {
+    canvas!.drawOval(rect, paint);
   }
 
   @override
@@ -1168,8 +1165,8 @@ class PaintDrawCircle extends DrawCommand {
   PaintDrawCircle(this.c, this.radius, this.paint);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.drawCircle(c, radius, paint);
+  void apply(EngineCanvas? canvas) {
+    canvas!.drawCircle(c, radius, paint);
   }
 
   @override
@@ -1200,8 +1197,8 @@ class PaintDrawPath extends DrawCommand {
   PaintDrawPath(this.path, this.paint);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.drawPath(path, paint);
+  void apply(EngineCanvas? canvas) {
+    canvas!.drawPath(path, paint);
   }
 
   @override
@@ -1233,8 +1230,8 @@ class PaintDrawShadow extends DrawCommand {
   final bool transparentOccluder;
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.drawShadow(path, color, elevation, transparentOccluder);
+  void apply(EngineCanvas? canvas) {
+    canvas!.drawShadow(path, color, elevation, transparentOccluder);
   }
 
   @override
@@ -1271,8 +1268,8 @@ class PaintDrawImage extends DrawCommand {
   PaintDrawImage(this.image, this.offset, this.paint);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.drawImage(image, offset, paint);
+  void apply(EngineCanvas? canvas) {
+    canvas!.drawImage(image, offset, paint);
   }
 
   @override
@@ -1301,8 +1298,8 @@ class PaintDrawImageRect extends DrawCommand {
   PaintDrawImageRect(this.image, this.src, this.dst, this.paint);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.drawImageRect(image, src, dst, paint);
+  void apply(EngineCanvas? canvas) {
+    canvas!.drawImageRect(image, src, dst, paint);
   }
 
   @override
@@ -1329,8 +1326,8 @@ class PaintDrawParagraph extends DrawCommand {
   PaintDrawParagraph(this.paragraph, this.offset);
 
   @override
-  void apply(EngineCanvas canvas) {
-    canvas.drawParagraph(paragraph, offset);
+  void apply(EngineCanvas? canvas) {
+    canvas!.drawParagraph(paragraph, offset);
   }
 
   @override
@@ -1351,7 +1348,7 @@ class PaintDrawParagraph extends DrawCommand {
 }
 
 List<dynamic> _serializePaintToCssPaint(SurfacePaintData paint) {
-  final EngineGradient engineShader = paint.shader;
+  final EngineGradient? engineShader = paint.shader as EngineGradient?;
   return <dynamic>[
     paint.blendMode?.index,
     paint.style?.index,
@@ -1573,11 +1570,7 @@ class Ellipse extends PathCommand {
         anticlockwise ? startAngle - endAngle : endAngle - startAngle,
         matrix4,
         bezierPath);
-    if (matrix4 != null) {
-      targetPath._addPathWithMatrix(bezierPath, 0, 0, matrix4);
-    } else {
-      targetPath._addPath(bezierPath, 0, 0);
-    }
+    targetPath._addPathWithMatrix(bezierPath as SurfacePath, 0, 0, matrix4);
   }
 
   void _drawArcWithBezier(
@@ -1855,11 +1848,7 @@ class RRectCommand extends PathCommand {
   void transform(Float32List matrix4, SurfacePath targetPath) {
     final ui.Path roundRectPath = ui.Path();
     _RRectToPathRenderer(roundRectPath).render(rrect);
-    if (matrix4 != null) {
-      targetPath._addPathWithMatrix(roundRectPath, 0, 0, matrix4);
-    } else {
-      targetPath._addPath(roundRectPath, 0, 0);
-    }
+    targetPath._addPathWithMatrix(roundRectPath as SurfacePath, 0, 0, matrix4);
   }
 
   @override
@@ -1908,11 +1897,15 @@ class _PaintBounds {
   // Bounds of actually painted area. If _left is not set, reported paintBounds
   // should be empty since growLTRB calls were outside active clipping
   // region.
-  double _left, _top, _right, _bottom;
+  double _left = double.maxFinite;
+  double _top = double.maxFinite;
+  double _right = -double.maxFinite;
+  double _bottom = -double.maxFinite;
+
   // Stack of transforms.
-  List<Matrix4> _transforms;
+  late List<Matrix4> _transforms = <Matrix4>[];
   // Stack of clip bounds.
-  List<ui.Rect> _clipStack;
+  late List<ui.Rect?> _clipStack = <ui.Rect?>[];
   bool _currentMatrixIsIdentity = true;
   Matrix4 _currentMatrix = Matrix4.identity();
   bool _clipRectInitialized = false;
@@ -1921,7 +1914,7 @@ class _PaintBounds {
       _currentClipRight = 0.0,
       _currentClipBottom = 0.0;
 
-  _PaintBounds(this.maxPaintBounds);
+  _PaintBounds(ui.Rect maxPaintBounds) : maxPaintBounds = maxPaintBounds;
 
   void translate(double dx, double dy) {
     if (dx != 0.0 || dy != 0.0) {
@@ -2100,9 +2093,7 @@ class _PaintBounds {
   }
 
   void saveTransformsAndClip() {
-    _clipStack ??= <ui.Rect>[];
-    _transforms ??= <Matrix4>[];
-    _transforms.add(_currentMatrix?.clone());
+    _transforms.add(_currentMatrix.clone());
     _clipStack.add(_clipRectInitialized
         ? ui.Rect.fromLTRB(_currentClipLeft, _currentClipTop, _currentClipRight,
             _currentClipBottom)
@@ -2111,7 +2102,7 @@ class _PaintBounds {
 
   void restoreTransformsAndClip() {
     _currentMatrix = _transforms.removeLast();
-    final ui.Rect clipRect = _clipStack.removeLast();
+    final ui.Rect? clipRect = _clipStack.removeLast();
     if (clipRect != null) {
       _currentClipLeft = clipRect.left;
       _currentClipTop = clipRect.top;
@@ -2176,14 +2167,14 @@ class _PaintBounds {
 /// boxes for paint operations that apply the paint.
 double _getPaintSpread(SurfacePaint paint) {
   double spread = 0.0;
-  final ui.MaskFilter maskFilter = paint?.maskFilter;
+  final ui.MaskFilter? maskFilter = paint.maskFilter;
   if (maskFilter != null) {
     // Multiply by 2 because the sigma is the standard deviation rather than
     // the length of the blur.
     // See also: https://developer.mozilla.org/en-US/docs/Web/CSS/filter-function/blur
     spread += maskFilter.webOnlySigma * 2.0;
   }
-  if (paint.strokeWidth != null && paint.strokeWidth != 0) {
+  if (paint.strokeWidth != 0) {
     // The multiplication by sqrt(2) is to account for line joints that
     // meet at 90-degree angle. Division by 2 is because only half of the
     // stroke is sticking out of the original shape. The other half is
diff --git a/lib/web_ui/lib/src/engine/surface/render_vertices.dart b/lib/web_ui/lib/src/engine/surface/render_vertices.dart
index 97ae71ec78f63..22ed41083b68b 100644
--- a/lib/web_ui/lib/src/engine/surface/render_vertices.dart
+++ b/lib/web_ui/lib/src/engine/surface/render_vertices.dart
@@ -2,24 +2,24 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
-_GlRenderer _glRenderer;
+_GlRenderer? _glRenderer;
 
 class SurfaceVertices implements ui.Vertices {
   final ui.VertexMode _mode;
   final Float32List _positions;
-  final Int32List/*?*/ _colors;
-  final Uint16List _indices; // ignore: unused_field
+  final Int32List? _colors;
+  final Uint16List? _indices; // ignore: unused_field
 
   SurfaceVertices(
     ui.VertexMode mode,
     List<ui.Offset> positions, {
-    List<ui.Color> colors,
-    List<int> indices,
-  })  : assert(mode != null),
-        assert(positions != null),
+    List<ui.Color>? colors,
+    List<int>? indices,
+  })  : assert(mode != null), // ignore: unnecessary_null_comparison
+        assert(positions != null), // ignore: unnecessary_null_comparison
         _mode = mode,
         _colors = colors != null ? _int32ListFromColors(colors) : null,
         _indices = indices != null ? Uint16List.fromList(indices) : null,
@@ -30,10 +30,10 @@ class SurfaceVertices implements ui.Vertices {
   SurfaceVertices.raw(
     ui.VertexMode mode,
     Float32List positions, {
-    Int32List colors,
-    Uint16List indices,
-  })  : assert(mode != null),
-        assert(positions != null),
+    Int32List? colors,
+    Uint16List? indices,
+  })  : assert(mode != null), // ignore: unnecessary_null_comparison
+        assert(positions != null), // ignore: unnecessary_null_comparison
         _mode = mode,
         _positions = positions,
         _colors = colors,
@@ -61,7 +61,7 @@ void disposeWebGl() {
 
 abstract class _GlRenderer {
   void drawVertices(
-      html.CanvasRenderingContext2D context,
+      html.CanvasRenderingContext2D? context,
       int canvasWidthInPixels,
       int canvasHeightInPixels,
       Matrix4 transform,
@@ -69,7 +69,7 @@ abstract class _GlRenderer {
       ui.BlendMode blendMode,
       SurfacePaintData paint);
 
-  void drawHairline(html.CanvasRenderingContext2D _ctx, Float32List positions);
+  void drawHairline(html.CanvasRenderingContext2D? _ctx, Float32List positions);
 }
 
 /// Treeshakeable backend for rendering webgl on canvas.
@@ -124,7 +124,7 @@ class _WebGlRenderer implements _GlRenderer {
 
   @override
   void drawVertices(
-      html.CanvasRenderingContext2D context,
+      html.CanvasRenderingContext2D? context,
       int canvasWidthInPixels,
       int canvasHeightInPixels,
       Matrix4 transform,
@@ -162,59 +162,59 @@ class _WebGlRenderer implements _GlRenderer {
       return;
     }
     _GlContext gl =
-        _OffscreenCanvas.createGlContext(widthInPixels, heightInPixels);
+        _OffscreenCanvas.createGlContext(widthInPixels, heightInPixels)!;
     final bool isWebKit = (browserEngine == BrowserEngine.webkit);
     _GlProgram glProgram = isWebKit
         ? gl.useAndCacheProgram(
-            _vertexShaderTriangleEs1, _fragmentShaderTriangleEs1)
+            _vertexShaderTriangleEs1, _fragmentShaderTriangleEs1)!
         : gl.useAndCacheProgram(
-            _vertexShaderTriangle, _fragmentShaderTriangle);
+            _vertexShaderTriangle, _fragmentShaderTriangle)!;
 
-    Object transformUniform = gl.getUniformLocation(glProgram.program, 'u_ctransform');
+    Object? transformUniform = gl.getUniformLocation(glProgram.program, 'u_ctransform');
     Matrix4 transformAtOffset = transform.clone()..translate(-offsetX, -offsetY);
     gl.setUniformMatrix4fv(transformUniform, false, transformAtOffset.storage);
 
     // Set uniform to scale 0..width/height pixels coordinates to -1..1
     // clipspace range and flip the Y axis.
-    Object resolution = gl.getUniformLocation(glProgram.program, 'u_scale');
+    Object? resolution = gl.getUniformLocation(glProgram.program, 'u_scale');
     gl.setUniform4f(resolution, 2.0 / widthInPixels.toDouble(),
         -2.0 / heightInPixels.toDouble(), 1, 1);
-    Object shift = gl.getUniformLocation(glProgram.program, 'u_shift');
+    Object? shift = gl.getUniformLocation(glProgram.program, 'u_shift');
     gl.setUniform4f(shift, -1, 1, 0, 0);
 
     // Setup geometry.
-    Object positionsBuffer = gl.createBuffer();
-    assert(positionsBuffer != null);
+    Object positionsBuffer = gl.createBuffer()!;
+    assert(positionsBuffer != null); // ignore: unnecessary_null_comparison
     gl.bindArrayBuffer(positionsBuffer);
     gl.bufferData(positions, gl.kStaticDraw);
     js_util.callMethod(
-        gl.glContext, 'vertexAttribPointer', <dynamic>[0, 2, gl.kFloat, false, 0, 0]);
+        gl.glContext!, 'vertexAttribPointer', <dynamic>[0, 2, gl.kFloat, false, 0, 0]);
     gl.enableVertexAttribArray(0);
 
     // Setup color buffer.
-    Object colorsBuffer = gl.createBuffer();
+    Object? colorsBuffer = gl.createBuffer();
     gl.bindArrayBuffer(colorsBuffer);
     // Buffer kBGRA_8888.
     gl.bufferData(vertices._colors, gl.kStaticDraw);
 
-    js_util.callMethod(gl.glContext, 'vertexAttribPointer',
+    js_util.callMethod(gl.glContext!, 'vertexAttribPointer',
         <dynamic>[1, 4, gl.kUnsignedByte, true, 0, 0]);
     gl.enableVertexAttribArray(1);
     gl.clear();
     final int vertexCount = positions.length ~/ 2;
     gl.drawTriangles(vertexCount, vertices._mode);
 
-    context.save();
+    context!.save();
     context.resetTransform();
     gl.drawImage(context, offsetX, offsetY);
     context.restore();
   }
 
   @override
-  void drawHairline(html.CanvasRenderingContext2D _ctx, Float32List positions) {
-    assert(positions != null);
+  void drawHairline(html.CanvasRenderingContext2D? _ctx, Float32List positions) {
+    assert(positions != null); // ignore: unnecessary_null_comparison
     final int pointCount = positions.length ~/ 2;
-    _ctx.lineWidth = 1.0;
+    _ctx!.lineWidth = 1.0;
     _ctx.beginPath();
     for (int i = 0, len = pointCount * 2; i < len;) {
       for (int triangleVertexIndex = 0;
@@ -337,13 +337,13 @@ Float32List _convertVertexPositions(ui.VertexMode mode, Float32List positions) {
 
 /// Compiled and cached gl program.
 class _GlProgram {
-  final Object program;
+  final Object? program;
   _GlProgram(this.program);
 }
 
 /// JS Interop helper for webgl apis.
 class _GlContext {
-  final Object glContext;
+  final Object? glContext;
   final bool isOffscreen;
   dynamic _kCompileStatus;
   dynamic _kArrayBuffer;
@@ -354,15 +354,15 @@ class _GlContext {
   dynamic _kLinkStatus;
   dynamic _kUnsignedByte;
   dynamic _kRGBA;
-  Object _canvas;
-  int _widthInPixels;
-  int _heightInPixels;
-  static Map<String, _GlProgram> _programCache;
+  Object? _canvas;
+  int? _widthInPixels;
+  int? _heightInPixels;
+  static late Map<String, _GlProgram?> _programCache;
 
   _GlContext.fromOffscreenCanvas(html.OffscreenCanvas canvas)
       : glContext = canvas.getContext('webgl2', <String, dynamic>{'premultipliedAlpha': false}),
         isOffscreen = true {
-    _programCache = <String, _GlProgram>{};
+    _programCache = <String, _GlProgram?>{};
     _canvas = canvas;
   }
 
@@ -370,7 +370,7 @@ class _GlContext {
       : glContext = canvas.getContext(useWebGl1 ? 'webgl' : 'webgl2',
           <String, dynamic>{'premultipliedAlpha': false}),
         isOffscreen = false {
-    _programCache = <String, _GlProgram>{};
+    _programCache = <String, _GlProgram?>{};
     _canvas = canvas;
   }
 
@@ -389,17 +389,17 @@ class _GlContext {
         left, top, _widthInPixels, _heightInPixels]);
   }
 
-  _GlProgram useAndCacheProgram(
+  _GlProgram? useAndCacheProgram(
       String vertexShaderSource, String fragmentShaderSource) {
     String cacheKey = '$vertexShaderSource||$fragmentShaderSource';
-    _GlProgram cachedProgram = _programCache[cacheKey];
+    _GlProgram? cachedProgram = _programCache[cacheKey];
     if (cachedProgram == null) {
       // Create and compile shaders.
       Object vertexShader = compileShader('VERTEX_SHADER', vertexShaderSource);
       Object fragmentShader =
           compileShader('FRAGMENT_SHADER', fragmentShaderSource);
       // Create a gl program and link shaders.
-      Object program = createProgram();
+      Object? program = createProgram();
       attachShader(program, vertexShader);
       attachShader(program, fragmentShader);
       linkProgram(program);
@@ -411,61 +411,61 @@ class _GlContext {
   }
 
   Object compileShader(String shaderType, String source) {
-    Object shader = _createShader(shaderType);
+    Object? shader = _createShader(shaderType);
     if (shader == null) {
       throw Exception(error);
     }
-    js_util.callMethod(glContext, 'shaderSource', <dynamic>[shader, source]);
-    js_util.callMethod(glContext, 'compileShader', <dynamic>[shader]);
+    js_util.callMethod(glContext!, 'shaderSource', <dynamic>[shader, source]);
+    js_util.callMethod(glContext!, 'compileShader', <dynamic>[shader]);
     bool shaderStatus = js_util
-        .callMethod(glContext, 'getShaderParameter', <dynamic>[shader, compileStatus]);
+        .callMethod(glContext!, 'getShaderParameter', <dynamic>[shader, compileStatus]);
     if (!shaderStatus) {
       throw Exception('Shader compilation failed: ${getShaderInfoLog(shader)}');
     }
     return shader;
   }
 
-  Object createProgram() =>
-      js_util.callMethod(glContext, 'createProgram', const <dynamic>[]);
+  Object? createProgram() =>
+      js_util.callMethod(glContext!, 'createProgram', const <dynamic>[]);
 
-  void attachShader(Object program, Object shader) {
-    js_util.callMethod(glContext, 'attachShader', <dynamic>[program, shader]);
+  void attachShader(Object? program, Object shader) {
+    js_util.callMethod(glContext!, 'attachShader', <dynamic>[program, shader]);
   }
 
-  void linkProgram(Object program) {
-    js_util.callMethod(glContext, 'linkProgram', <dynamic>[program]);
+  void linkProgram(Object? program) {
+    js_util.callMethod(glContext!, 'linkProgram', <dynamic>[program]);
     if (!js_util
-        .callMethod(glContext, 'getProgramParameter', <dynamic>[program, kLinkStatus])) {
+        .callMethod(glContext!, 'getProgramParameter', <dynamic>[program, kLinkStatus])) {
       throw Exception(getProgramInfoLog(program));
     }
   }
 
-  void useProgram(Object program) {
-    js_util.callMethod(glContext, 'useProgram', <dynamic>[program]);
+  void useProgram(Object? program) {
+    js_util.callMethod(glContext!, 'useProgram', <dynamic>[program]);
   }
 
-  Object createBuffer() =>
-      js_util.callMethod(glContext, 'createBuffer', const <dynamic>[]);
+  Object? createBuffer() =>
+      js_util.callMethod(glContext!, 'createBuffer', const <dynamic>[]);
 
-  void bindArrayBuffer(Object buffer) {
-    js_util.callMethod(glContext, 'bindBuffer', <dynamic>[kArrayBuffer, buffer]);
+  void bindArrayBuffer(Object? buffer) {
+    js_util.callMethod(glContext!, 'bindBuffer', <dynamic>[kArrayBuffer, buffer]);
   }
 
   void deleteBuffer(Object buffer) {
-    js_util.callMethod(glContext, 'deleteBuffer', <dynamic>[buffer]);
+    js_util.callMethod(glContext!, 'deleteBuffer', <dynamic>[buffer]);
   }
 
-  void bufferData(TypedData data, dynamic type) {
-    js_util.callMethod(glContext, 'bufferData', <dynamic>[kArrayBuffer, data, type]);
+  void bufferData(TypedData? data, dynamic type) {
+    js_util.callMethod(glContext!, 'bufferData', <dynamic>[kArrayBuffer, data, type]);
   }
 
   void enableVertexAttribArray(int index) {
-    js_util.callMethod(glContext, 'enableVertexAttribArray', <dynamic>[index]);
+    js_util.callMethod(glContext!, 'enableVertexAttribArray', <dynamic>[index]);
   }
 
   /// Clear background.
   void clear() {
-    js_util.callMethod(glContext, 'clear', <dynamic>[kColorBufferBit]);
+    js_util.callMethod(glContext!, 'clear', <dynamic>[kColorBufferBit]);
   }
 
   /// Destroys gl context.
@@ -474,25 +474,25 @@ class _GlContext {
   }
 
   void deleteProgram(Object program) {
-    js_util.callMethod(glContext, 'deleteProgram', <dynamic>[program]);
+    js_util.callMethod(glContext!, 'deleteProgram', <dynamic>[program]);
   }
 
   void deleteShader(Object shader) {
-    js_util.callMethod(glContext, 'deleteShader', <dynamic>[shader]);
+    js_util.callMethod(glContext!, 'deleteShader', <dynamic>[shader]);
   }
 
   dynamic _getExtension(String extensionName) =>
-      js_util.callMethod(glContext, 'getExtension', <dynamic>[extensionName]);
+      js_util.callMethod(glContext!, 'getExtension', <dynamic>[extensionName]);
 
   void drawTriangles(int triangleCount, ui.VertexMode vertexMode) {
     dynamic mode = _triangleTypeFromMode(vertexMode);
-    js_util.callMethod(glContext, 'drawArrays', <dynamic>[mode, 0, triangleCount]);
+    js_util.callMethod(glContext!, 'drawArrays', <dynamic>[mode, 0, triangleCount]);
   }
 
   /// Sets affine transformation from normalized device coordinates
   /// to window coordinates
   void viewport(double x, double y, double width, double height) {
-    js_util.callMethod(glContext, 'viewport', <dynamic>[x, y, width, height]);
+    js_util.callMethod(glContext!, 'viewport', <dynamic>[x, y, width, height]);
   }
 
   dynamic _triangleTypeFromMode(ui.VertexMode mode) {
@@ -509,105 +509,105 @@ class _GlContext {
     }
   }
 
-  Object _createShader(String shaderType) => js_util.callMethod(
-      glContext, 'createShader', <dynamic>[js_util.getProperty(glContext, shaderType)]);
+  Object? _createShader(String shaderType) => js_util.callMethod(
+      glContext!, 'createShader', <dynamic>[js_util.getProperty(glContext!, shaderType)]);
 
   /// Error state of gl context.
-  dynamic get error => js_util.callMethod(glContext, 'getError', const <dynamic>[]);
+  dynamic get error => js_util.callMethod(glContext!, 'getError', const <dynamic>[]);
 
   /// Shader compiler error, if this returns [kFalse], to get details use
   /// [getShaderInfoLog].
   dynamic get compileStatus =>
-      _kCompileStatus ??= js_util.getProperty(glContext, 'COMPILE_STATUS');
+      _kCompileStatus ??= js_util.getProperty(glContext!, 'COMPILE_STATUS');
 
   dynamic get kArrayBuffer =>
-      _kArrayBuffer ??= js_util.getProperty(glContext, 'ARRAY_BUFFER');
+      _kArrayBuffer ??= js_util.getProperty(glContext!, 'ARRAY_BUFFER');
 
   dynamic get kLinkStatus =>
-      _kLinkStatus ??= js_util.getProperty(glContext, 'LINK_STATUS');
+      _kLinkStatus ??= js_util.getProperty(glContext!, 'LINK_STATUS');
 
-  dynamic get kFloat => _kFloat ??= js_util.getProperty(glContext, 'FLOAT');
+  dynamic get kFloat => _kFloat ??= js_util.getProperty(glContext!, 'FLOAT');
 
-  dynamic get kRGBA => _kRGBA ??= js_util.getProperty(glContext, 'RGBA');
+  dynamic get kRGBA => _kRGBA ??= js_util.getProperty(glContext!, 'RGBA');
 
   dynamic get kUnsignedByte =>
-      _kUnsignedByte ??= js_util.getProperty(glContext, 'UNSIGNED_BYTE');
+      _kUnsignedByte ??= js_util.getProperty(glContext!, 'UNSIGNED_BYTE');
 
   dynamic get kStaticDraw =>
-      _kStaticDraw ??= js_util.getProperty(glContext, 'STATIC_DRAW');
+      _kStaticDraw ??= js_util.getProperty(glContext!, 'STATIC_DRAW');
 
   dynamic get kTriangles =>
-      _kTriangles ??= js_util.getProperty(glContext, 'TRIANGLES');
+      _kTriangles ??= js_util.getProperty(glContext!, 'TRIANGLES');
 
   dynamic get kTriangleFan =>
-      _kTriangles ??= js_util.getProperty(glContext, 'TRIANGLE_FAN');
+      _kTriangles ??= js_util.getProperty(glContext!, 'TRIANGLE_FAN');
 
   dynamic get kTriangleStrip =>
-      _kTriangles ??= js_util.getProperty(glContext, 'TRIANGLE_STRIP');
+      _kTriangles ??= js_util.getProperty(glContext!, 'TRIANGLE_STRIP');
 
   dynamic get kColorBufferBit =>
-      _kColorBufferBit ??= js_util.getProperty(glContext, 'COLOR_BUFFER_BIT');
+      _kColorBufferBit ??= js_util.getProperty(glContext!, 'COLOR_BUFFER_BIT');
 
   /// Returns reference to uniform in program.
-  Object getUniformLocation(Object program, String uniformName) {
+  Object? getUniformLocation(Object? program, String uniformName) {
     return js_util
-        .callMethod(glContext, 'getUniformLocation', <dynamic>[program, uniformName]);
+        .callMethod(glContext!, 'getUniformLocation', <dynamic>[program, uniformName]);
   }
 
   /// Sets vec2 uniform values.
   void setUniform2f(Object uniform, double value1, double value2) {
     return js_util
-        .callMethod(glContext, 'uniform2f', <dynamic>[uniform, value1, value2]);
+        .callMethod(glContext!, 'uniform2f', <dynamic>[uniform, value1, value2]);
   }
 
   /// Sets vec4 uniform values.
-  void setUniform4f(Object uniform, double value1, double value2, double value3,
+  void setUniform4f(Object? uniform, double value1, double value2, double value3,
       double value4) {
     return js_util.callMethod(
-        glContext, 'uniform4f', <dynamic>[uniform, value1, value2, value3, value4]);
+        glContext!, 'uniform4f', <dynamic>[uniform, value1, value2, value3, value4]);
   }
 
   /// Sets mat4 uniform values.
-  void setUniformMatrix4fv(Object uniform, bool transpose, Float32List value) {
+  void setUniformMatrix4fv(Object? uniform, bool transpose, Float32List? value) {
     return js_util.callMethod(
-        glContext, 'uniformMatrix4fv', <dynamic>[uniform, transpose, value]);
+        glContext!, 'uniformMatrix4fv', <dynamic>[uniform, transpose, value]);
   }
 
   /// Shader compile error log.
   dynamic getShaderInfoLog(Object glShader) {
-    return js_util.callMethod(glContext, 'getShaderInfoLog', <dynamic>[glShader]);
+    return js_util.callMethod(glContext!, 'getShaderInfoLog', <dynamic>[glShader]);
   }
 
   ///  Errors that occurred during failed linking or validation of program
   ///  objects. Typically called after [linkProgram].
-  String getProgramInfoLog(Object glProgram) {
-    return js_util.callMethod(glContext, 'getProgramInfoLog', <dynamic>[glProgram]);
+  String? getProgramInfoLog(Object? glProgram) {
+    return js_util.callMethod(glContext!, 'getProgramInfoLog', <dynamic>[glProgram]);
   }
 
-  int get drawingBufferWidth =>
-      js_util.getProperty(glContext, 'drawingBufferWidth');
-  int get drawingBufferHeight =>
-      js_util.getProperty(glContext, 'drawingBufferWidth');
+  int? get drawingBufferWidth =>
+      js_util.getProperty(glContext!, 'drawingBufferWidth');
+  int? get drawingBufferHeight =>
+      js_util.getProperty(glContext!, 'drawingBufferWidth');
 
   html.ImageData readImageData() {
     if (browserEngine == BrowserEngine.webkit ||
         browserEngine == BrowserEngine.firefox) {
       const int kBytesPerPixel = 4;
-      final int bufferWidth = _widthInPixels;
-      final int bufferHeight = _heightInPixels;
+      final int bufferWidth = _widthInPixels!;
+      final int bufferHeight = _heightInPixels!;
       final Uint8List pixels =
           Uint8List(bufferWidth * bufferHeight * kBytesPerPixel);
-      js_util.callMethod(glContext, 'readPixels',
+      js_util.callMethod(glContext!, 'readPixels',
           <dynamic>[0, 0, bufferWidth, bufferHeight, kRGBA, kUnsignedByte, pixels]);
       return html.ImageData(
           Uint8ClampedList.fromList(pixels), bufferWidth, bufferHeight);
     } else {
       const int kBytesPerPixel = 4;
-      final int bufferWidth = _widthInPixels;
-      final int bufferHeight = _heightInPixels;
+      final int bufferWidth = _widthInPixels!;
+      final int bufferHeight = _heightInPixels!;
       final Uint8ClampedList pixels =
           Uint8ClampedList(bufferWidth * bufferHeight * kBytesPerPixel);
-      js_util.callMethod(glContext, 'readPixels',
+      js_util.callMethod(glContext!, 'readPixels',
           <dynamic>[0, 0, bufferWidth, bufferHeight, kRGBA, kUnsignedByte, pixels]);
       return html.ImageData(pixels, bufferWidth, bufferHeight);
     }
@@ -616,11 +616,11 @@ class _GlContext {
 
 /// Shared Cached OffscreenCanvas for webgl rendering to image.
 class _OffscreenCanvas {
-  static html.OffscreenCanvas _canvas;
+  static html.OffscreenCanvas? _canvas;
   static int _maxPixelWidth = 0;
   static int _maxPixelHeight = 0;
-  static html.CanvasElement _glCanvas;
-  static _GlContext _cachedContext;
+  static html.CanvasElement? _glCanvas;
+  static _GlContext? _cachedContext;
 
   _OffscreenCanvas(int width, int height) {
     assert(width > 0 && height > 0);
@@ -642,16 +642,16 @@ class _OffscreenCanvas {
     _cachedContext = null;
   }
 
-  html.OffscreenCanvas get canvas => _canvas;
+  html.OffscreenCanvas? get canvas => _canvas;
 
-  static _GlContext createGlContext(int widthInPixels, int heightInPixels) {
+  static _GlContext? createGlContext(int widthInPixels, int heightInPixels) {
     final bool isWebKit = (browserEngine == BrowserEngine.webkit);
 
     if (_OffscreenCanvas.supported) {
       final _OffscreenCanvas offScreenCanvas =
           _OffscreenCanvas(widthInPixels, heightInPixels);
-      _cachedContext ??= _GlContext.fromOffscreenCanvas(offScreenCanvas.canvas);
-      _cachedContext.setViewportSize(widthInPixels, heightInPixels);
+      _cachedContext ??= _GlContext.fromOffscreenCanvas(offScreenCanvas.canvas!);
+      _cachedContext!.setViewportSize(widthInPixels, heightInPixels);
       return _cachedContext;
     } else {
       // Allocate new canvas element is size is larger.
@@ -660,10 +660,10 @@ class _OffscreenCanvas {
           width: widthInPixels,
           height: heightInPixels,
         );
-        _glCanvas.className = 'gl-canvas';
+        _glCanvas!.className = 'gl-canvas';
         final double cssWidth = widthInPixels / EngineWindow.browserDevicePixelRatio;
         final double cssHeight = heightInPixels / EngineWindow.browserDevicePixelRatio;
-        _glCanvas.style
+        _glCanvas!.style
           ..position = 'absolute'
           ..width = '${cssWidth}px'
           ..height = '${cssHeight}px';
@@ -672,8 +672,8 @@ class _OffscreenCanvas {
         _cachedContext?.dispose();
         _cachedContext = null;
       }
-      _cachedContext ??= _GlContext.fromCanvas(_glCanvas, isWebKit);
-      _cachedContext.setViewportSize(widthInPixels, heightInPixels);
+      _cachedContext ??= _GlContext.fromCanvas(_glCanvas!, isWebKit);
+      _cachedContext!.setViewportSize(widthInPixels, heightInPixels);
       return _cachedContext;
     }
   }
diff --git a/lib/web_ui/lib/src/engine/surface/scene.dart b/lib/web_ui/lib/src/engine/surface/scene.dart
index 832b3fd9eb68c..f1c6afcbb98ef 100644
--- a/lib/web_ui/lib/src/engine/surface/scene.dart
+++ b/lib/web_ui/lib/src/engine/surface/scene.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 class SurfaceScene implements ui.Scene {
@@ -12,7 +12,7 @@ class SurfaceScene implements ui.Scene {
   /// To create a Scene object, use a [SceneBuilder].
   SurfaceScene(this.webOnlyRootElement);
 
-  final html.Element webOnlyRootElement;
+  final html.Element? webOnlyRootElement;
 
   /// Creates a raster image representation of the current state of the scene.
   /// This is a slow operation that is performed on a background thread.
@@ -28,7 +28,7 @@ class SurfaceScene implements ui.Scene {
 
 /// A surface that creates a DOM element for whole app.
 class PersistedScene extends PersistedContainerSurface {
-  PersistedScene(PersistedScene oldLayer) : super(oldLayer) {
+  PersistedScene(PersistedScene? oldLayer) : super(oldLayer) {
     _transform = Matrix4.identity();
   }
 
@@ -46,7 +46,7 @@ class PersistedScene extends PersistedContainerSurface {
   }
 
   @override
-  Matrix4 get localTransformInverse => _localTransformInverse;
+  Matrix4? get localTransformInverse => _localTransformInverse;
 
   @override
   html.Element createElement() {
diff --git a/lib/web_ui/lib/src/engine/surface/scene_builder.dart b/lib/web_ui/lib/src/engine/surface/scene_builder.dart
index 4a6ebaced0f72..d08d9ca07d79a 100644
--- a/lib/web_ui/lib/src/engine/surface/scene_builder.dart
+++ b/lib/web_ui/lib/src/engine/surface/scene_builder.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 class SurfaceSceneBuilder implements ui.SceneBuilder {
@@ -17,7 +17,7 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
   ///
   /// This getter should only be called after all surfaces are built.
   PersistedScene get _persistedScene {
-    return _surfaceStack.first;
+    return _surfaceStack.first as PersistedScene;
   }
 
   /// The surface currently being built.
@@ -27,10 +27,10 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
     // Only attempt to update if the update is requested and the surface is in
     // the live tree.
     if (surface.oldLayer != null) {
-      assert(surface.oldLayer.runtimeType == surface.runtimeType);
+      assert(surface.oldLayer!.runtimeType == surface.runtimeType);
       assert(debugAssertSurfaceState(
-          surface.oldLayer, PersistedSurfaceState.active));
-      surface.oldLayer.state = PersistedSurfaceState.pendingUpdate;
+          surface.oldLayer!, PersistedSurfaceState.active));
+      surface.oldLayer!.state = PersistedSurfaceState.pendingUpdate;
     }
     _adoptSurface(surface);
     _surfaceStack.add(surface);
@@ -63,9 +63,9 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
   ui.OffsetEngineLayer pushOffset(
     double dx,
     double dy, {
-    ui.OffsetEngineLayer oldLayer,
+    ui.OffsetEngineLayer? oldLayer,
   }) {
-    return _pushSurface(PersistedOffset(oldLayer, dx, dy));
+    return _pushSurface(PersistedOffset(oldLayer as PersistedOffset?, dx, dy)) as ui.OffsetEngineLayer;
   }
 
   /// Pushes a transform operation onto the operation stack.
@@ -76,9 +76,9 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
   @override
   ui.TransformEngineLayer pushTransform(
     Float64List matrix4, {
-    ui.TransformEngineLayer oldLayer,
+    ui.TransformEngineLayer? oldLayer,
   }) {
-    if (matrix4 == null) {
+    if (matrix4 == null) { // ignore: unnecessary_null_comparison
       throw ArgumentError('"matrix4" argument cannot be null');
     }
     if (matrix4.length != 16) {
@@ -86,7 +86,7 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
     }
 
     // TODO(yjbanov): make this final after NNBD ships definite assignment.
-    /*final*/ Float32List matrix;
+    /*final*/ Float32List? matrix;
     if (_surfaceStack.length == 1) {
       // Top level transform contains view configuration to scale
       // scene to devicepixelratio. Use identity instead since CSS uses
@@ -99,7 +99,7 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
     } else {
       matrix = toMatrix32(matrix4);
     }
-    return _pushSurface(PersistedTransform(oldLayer, matrix));
+    return _pushSurface(PersistedTransform(oldLayer as PersistedTransform?, matrix)) as ui.TransformEngineLayer;
   }
 
   /// Pushes a rectangular clip operation onto the operation stack.
@@ -112,11 +112,11 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
   ui.ClipRectEngineLayer pushClipRect(
     ui.Rect rect, {
     ui.Clip clipBehavior = ui.Clip.antiAlias,
-    ui.ClipRectEngineLayer oldLayer,
+    ui.ClipRectEngineLayer? oldLayer,
   }) {
-    assert(clipBehavior != null);
+    assert(clipBehavior != null); // ignore: unnecessary_null_comparison
     assert(clipBehavior != ui.Clip.none);
-    return _pushSurface(PersistedClipRect(oldLayer, rect));
+    return _pushSurface(PersistedClipRect(oldLayer as PersistedClipRect?, rect)) as ui.ClipRectEngineLayer;
   }
 
   /// Pushes a rounded-rectangular clip operation onto the operation stack.
@@ -127,10 +127,10 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
   @override
   ui.ClipRRectEngineLayer pushClipRRect(
     ui.RRect rrect, {
-    ui.Clip clipBehavior,
-    ui.ClipRRectEngineLayer oldLayer,
+    ui.Clip? clipBehavior,
+    ui.ClipRRectEngineLayer? oldLayer,
   }) {
-    return _pushSurface(PersistedClipRRect(oldLayer, rrect, clipBehavior));
+    return _pushSurface(PersistedClipRRect(oldLayer, rrect, clipBehavior)) as ui.ClipRRectEngineLayer;
   }
 
   /// Pushes a path clip operation onto the operation stack.
@@ -142,11 +142,11 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
   ui.ClipPathEngineLayer pushClipPath(
     ui.Path path, {
     ui.Clip clipBehavior = ui.Clip.antiAlias,
-    ui.ClipPathEngineLayer oldLayer,
+    ui.ClipPathEngineLayer? oldLayer,
   }) {
-    assert(clipBehavior != null);
+    assert(clipBehavior != null); // ignore: unnecessary_null_comparison
     assert(clipBehavior != ui.Clip.none);
-    return _pushSurface(PersistedClipPath(oldLayer, path, clipBehavior));
+    return _pushSurface(PersistedClipPath(oldLayer as PersistedClipPath?, path, clipBehavior)) as ui.ClipPathEngineLayer;
   }
 
   /// Pushes an opacity operation onto the operation stack.
@@ -161,9 +161,9 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
   ui.OpacityEngineLayer pushOpacity(
     int alpha, {
     ui.Offset offset = ui.Offset.zero,
-    ui.OpacityEngineLayer oldLayer,
+    ui.OpacityEngineLayer? oldLayer,
   }) {
-    return _pushSurface(PersistedOpacity(oldLayer, alpha, offset));
+    return _pushSurface(PersistedOpacity(oldLayer as PersistedOpacity?, alpha, offset)) as ui.OpacityEngineLayer;
   }
 
   /// Pushes a color filter operation onto the operation stack.
@@ -179,9 +179,9 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
   @override
   ui.ColorFilterEngineLayer pushColorFilter(
     ui.ColorFilter filter, {
-    ui.ColorFilterEngineLayer oldLayer,
+    ui.ColorFilterEngineLayer? oldLayer,
   }) {
-    assert(filter != null);
+    assert(filter != null); // ignore: unnecessary_null_comparison
     throw UnimplementedError();
   }
 
@@ -198,10 +198,10 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
   @override
   ui.ImageFilterEngineLayer pushImageFilter(
     ui.ImageFilter filter, {
-    ui.ImageFilterEngineLayer oldLayer,
+    ui.ImageFilterEngineLayer? oldLayer,
   }) {
-    assert(filter != null);
-    return _pushSurface(PersistedImageFilter(oldLayer, filter));
+    assert(filter != null); // ignore: unnecessary_null_comparison
+    return _pushSurface(PersistedImageFilter(oldLayer as PersistedImageFilter?, filter)) as ui.ImageFilterEngineLayer;
   }
 
   /// Pushes a backdrop filter operation onto the operation stack.
@@ -213,9 +213,9 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
   @override
   ui.BackdropFilterEngineLayer pushBackdropFilter(
     ui.ImageFilter filter, {
-    ui.BackdropFilterEngineLayer oldLayer,
+    ui.BackdropFilterEngineLayer? oldLayer,
   }) {
-    return _pushSurface(PersistedBackdropFilter(oldLayer, filter));
+    return _pushSurface(PersistedBackdropFilter(oldLayer as PersistedBackdropFilter?, filter as EngineImageFilter)) as ui.BackdropFilterEngineLayer;
   }
 
   /// Pushes a shader mask operation onto the operation stack.
@@ -229,7 +229,7 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
     ui.Shader shader,
     ui.Rect maskRect,
     ui.BlendMode blendMode, {
-    ui.ShaderMaskEngineLayer oldLayer,
+    ui.ShaderMaskEngineLayer? oldLayer,
   }) {
     throw UnimplementedError();
   }
@@ -248,22 +248,22 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
   /// See [pop] for details about the operation stack, and [Clip] for different clip modes.
   @override
   ui.PhysicalShapeEngineLayer pushPhysicalShape({
-    ui.Path path,
-    double elevation,
-    ui.Color color,
-    ui.Color shadowColor,
+    required ui.Path path,
+    required double elevation,
+    required ui.Color color,
+    ui.Color? shadowColor,
     ui.Clip clipBehavior = ui.Clip.none,
-    ui.PhysicalShapeEngineLayer oldLayer,
+    ui.PhysicalShapeEngineLayer? oldLayer,
   }) {
-    assert(color != null, 'color must not be null');
+    assert(color != null, 'color must not be null'); // ignore: unnecessary_null_comparison
     return _pushSurface(PersistedPhysicalShape(
-      oldLayer,
-      path,
+      oldLayer as PersistedPhysicalShape?,
+      path as SurfacePath,
       elevation,
       color.value,
       shadowColor?.value ?? 0xFF000000,
       clipBehavior,
-    ));
+    )) as ui.PhysicalShapeEngineLayer;
   }
 
   /// Add a retained engine layer subtree from previous frames.
@@ -276,7 +276,7 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
   /// no need to call [addToScene] for its children layers.
   @override
   void addRetained(ui.EngineLayer retainedLayer) {
-    final PersistedContainerSurface retainedSurface = retainedLayer;
+    final PersistedContainerSurface retainedSurface = retainedLayer as PersistedContainerSurface;
     if (assertionsEnabled) {
       assert(debugAssertSurfaceState(retainedSurface,
           PersistedSurfaceState.active, PersistedSurfaceState.released));
@@ -379,7 +379,7 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
     bool freeze = false,
     ui.FilterQuality filterQuality = ui.FilterQuality.low,
   }) {
-    assert(offset != null, 'Offset argument was null');
+    assert(offset != null, 'Offset argument was null'); // ignore: unnecessary_null_comparison
     _addTexture(offset.dx, offset.dy, width, height, textureId, filterQuality.index);
   }
 
@@ -414,7 +414,7 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
     double width = 0.0,
     double height = 0.0,
   }) {
-    assert(offset != null, 'Offset argument was null');
+    assert(offset != null, 'Offset argument was null'); // ignore: unnecessary_null_comparison
     _addPlatformView(offset.dx, offset.dy, width, height, viewId);
   }
 
@@ -435,7 +435,7 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
     ui.Offset offset = ui.Offset.zero,
     double width = 0.0,
     double height = 0.0,
-    ui.SceneHost sceneHost,
+    ui.SceneHost? sceneHost,
     bool hitTestable = true,
   }) {
     _addChildScene(offset.dx, offset.dy, width, height, sceneHost, hitTestable);
@@ -446,7 +446,7 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
     double dy,
     double width,
     double height,
-    ui.SceneHost sceneHost,
+    ui.SceneHost? sceneHost,
     bool hitTestable,
   ) {
     throw UnimplementedError();
@@ -492,14 +492,14 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
   ///
   /// This is a surface tree that holds onto the DOM elements that can be reused
   /// on the next frame.
-  static PersistedScene _lastFrameScene;
+  static PersistedScene? _lastFrameScene;
 
   /// Returns the computed persisted scene graph recorded in the last frame.
   ///
   /// This is only available in debug mode. It returns `null` in profile and
   /// release modes.
-  static PersistedScene get debugLastFrameScene {
-    PersistedScene result;
+  static PersistedScene? get debugLastFrameScene {
+    PersistedScene? result;
     assert(() {
       result = _lastFrameScene;
       return true;
@@ -542,7 +542,7 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
       if (_lastFrameScene == null) {
         _persistedScene.build();
       } else {
-        _persistedScene.update(_lastFrameScene);
+        _persistedScene.update(_lastFrameScene!);
       }
       commitScene(_persistedScene);
       _lastFrameScene = _persistedScene;
diff --git a/lib/web_ui/lib/src/engine/surface/surface.dart b/lib/web_ui/lib/src/engine/surface/surface.dart
index dce4664e3d64b..0562f6e00ce98 100644
--- a/lib/web_ui/lib/src/engine/surface/surface.dart
+++ b/lib/web_ui/lib/src/engine/surface/surface.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// When `true` prints statistics about what happened to the surface tree when
@@ -165,8 +165,8 @@ class PersistedSurfaceException implements Exception {
 /// This function should be used inside an assertion expression.
 bool debugAssertSurfaceState(
     PersistedSurface surface, PersistedSurfaceState state1,
-    [PersistedSurfaceState state2, PersistedSurfaceState state3]) {
-  final List<PersistedSurfaceState> validStates = [state1, state2, state3];
+    [PersistedSurfaceState? state2, PersistedSurfaceState? state3]) {
+  final List<PersistedSurfaceState?> validStates = [state1, state2, state3];
 
   if (validStates.contains(surface.state)) {
     return true;
@@ -184,7 +184,7 @@ bool debugAssertSurfaceState(
 /// compute the fewest amount of mutations necessary to update the browser DOM.
 abstract class PersistedSurface implements ui.EngineLayer {
   /// Creates a persisted surface.
-  PersistedSurface(PersistedSurface oldLayer)
+  PersistedSurface(PersistedSurface? oldLayer)
     : _oldLayer = FrameReference<PersistedSurface>(
             oldLayer != null && oldLayer.isActive ? oldLayer : null);
 
@@ -193,7 +193,7 @@ abstract class PersistedSurface implements ui.EngineLayer {
   /// If not null this surface will reuse the old surface's HTML [element].
   ///
   /// This value is set to null at the end of the frame.
-  PersistedSurface get oldLayer => _oldLayer.value;
+  PersistedSurface? get oldLayer => _oldLayer.value;
   final FrameReference<PersistedSurface> _oldLayer;
 
   /// The index of this surface in its parent's [PersistedContainerSurface._children]
@@ -210,7 +210,7 @@ abstract class PersistedSurface implements ui.EngineLayer {
   /// surface.
   PersistedSurfaceState get state => _state;
   set state(PersistedSurfaceState newState) {
-    assert(newState != null);
+    assert(newState != null); // ignore: unnecessary_null_comparison
     assert(newState != _state,
         'Attempted to set state that the surface is already in. This likely indicates a bug in the compositor.');
     assert(_debugValidateStateTransition(newState));
@@ -301,7 +301,7 @@ abstract class PersistedSurface implements ui.EngineLayer {
   ///
   /// This element can be reused across frames. See also, [childContainer],
   /// which is the element used to manage child nodes.
-  html.Element rootElement;
+  html.Element? rootElement;
 
   /// Whether this surface can update an existing [oldSurface].
   bool canUpdateAsMatch(PersistedSurface oldSurface) {
@@ -313,10 +313,10 @@ abstract class PersistedSurface implements ui.EngineLayer {
   /// By default this is the same as the [rootElement]. However, specialized
   /// surface implementations may choose to override this and provide a
   /// different element for nesting children.
-  html.Element get childContainer => rootElement;
+  html.Element? get childContainer => rootElement;
 
   /// This surface's immediate parent.
-  PersistedContainerSurface parent;
+  PersistedContainerSurface? parent;
 
   /// Visits immediate children.
   ///
@@ -336,7 +336,7 @@ abstract class PersistedSurface implements ui.EngineLayer {
   /// compared to higher scores. For example, when matching a picture with a
   /// bitmap canvas the score is higher for a canvas that's bigger in size than
   /// a smaller canvas that also fits the picture.
-  double matchForUpdate(covariant PersistedSurface existingSurface);
+  double matchForUpdate(covariant PersistedSurface? existingSurface);
 
   /// Creates a new element and sets the necessary HTML and CSS attributes.
   ///
@@ -347,10 +347,10 @@ abstract class PersistedSurface implements ui.EngineLayer {
   void build() {
     if (rootElement != null) {
       try {
-        throw null;
+        throw '';
       } catch (_, stack) {
         print(
-            'Attempted to build a $runtimeType, but it already has an HTML element ${rootElement.tagName}.');
+            'Attempted to build a $runtimeType, but it already has an HTML element ${rootElement!.tagName}.');
         print(stack.toString().split('\n').take(20).join('\n'));
       }
     }
@@ -379,7 +379,7 @@ abstract class PersistedSurface implements ui.EngineLayer {
         PersistedSurfaceState.pendingUpdate));
     assert(() {
       if (oldSurface.isPendingUpdate) {
-        final PersistedContainerSurface self = this;
+        final PersistedContainerSurface self = this as PersistedContainerSurface;
         assert(identical(self.oldLayer, oldSurface));
       }
       return true;
@@ -402,7 +402,7 @@ abstract class PersistedSurface implements ui.EngineLayer {
   @protected
   @mustCallSuper
   void update(covariant PersistedSurface oldSurface) {
-    assert(oldSurface != null);
+    assert(oldSurface != null); // ignore: unnecessary_null_comparison
     assert(!identical(oldSurface, this));
     assert(debugAssertSurfaceState(this, PersistedSurfaceState.created));
     assert(debugAssertSurfaceState(oldSurface, PersistedSurfaceState.active,
@@ -411,7 +411,7 @@ abstract class PersistedSurface implements ui.EngineLayer {
     adoptElements(oldSurface);
 
     if (assertionsEnabled) {
-      rootElement.setAttribute('flt-layer-state', 'updated');
+      rootElement!.setAttribute('flt-layer-state', 'updated');
     }
     state = PersistedSurfaceState.active;
     assert(rootElement != null);
@@ -436,7 +436,7 @@ abstract class PersistedSurface implements ui.EngineLayer {
       _retainedSurfaces.add(this);
     }
     if (assertionsEnabled) {
-      rootElement.setAttribute('flt-layer-state', 'retained');
+      rootElement!.setAttribute('flt-layer-state', 'retained');
     }
     if (_debugExplainSurfaceStats) {
       _surfaceStatsFor(this).retainSurfaceCount++;
@@ -466,7 +466,7 @@ abstract class PersistedSurface implements ui.EngineLayer {
     //                the DOM, and the second method will disassociate the
     //                surface from the DOM and release it irrespective of
     //                whether the DOM itself gets detached or not.
-    rootElement.remove();
+    rootElement!.remove();
     rootElement = null;
     state = PersistedSurfaceState.released;
   }
@@ -510,8 +510,8 @@ abstract class PersistedSurface implements ui.EngineLayer {
   /// transforms as well as this layer's transform (if any).
   ///
   /// The value is update by [recomputeTransformAndClip].
-  Matrix4 get transform => _transform;
-  Matrix4 _transform;
+  Matrix4? get transform => _transform;
+  Matrix4? _transform;
 
   /// The intersection at this surface level.
   ///
@@ -519,19 +519,19 @@ abstract class PersistedSurface implements ui.EngineLayer {
   /// the clip added by this layer (if any).
   ///
   /// The value is update by [recomputeTransformAndClip].
-  ui.Rect _projectedClip;
+  ui.Rect? _projectedClip;
 
   /// Bounds of clipping performed by this layer.
-  ui.Rect _localClipBounds;
+  ui.Rect? _localClipBounds;
   // Cached inverse of transform on this node. Unlike transform, this
   // Matrix only contains local transform (not chain multiplied since root).
-  Matrix4 _localTransformInverse;
+  Matrix4? _localTransformInverse;
 
   /// The inverse of the local transform that this surface applies to its children.
   ///
   /// The default implementation is identity transform. Concrete
   /// implementations may override this getter to supply a different transform.
-  Matrix4 get localTransformInverse =>
+  Matrix4? get localTransformInverse =>
       _localTransformInverse ??= Matrix4.identity();
 
   /// Recomputes [transform] and [globalClip] fields.
@@ -543,7 +543,7 @@ abstract class PersistedSurface implements ui.EngineLayer {
   /// This method is called by the [preroll] method.
   @protected
   void recomputeTransformAndClip() {
-    _transform = parent._transform;
+    _transform = parent!._transform;
     _localClipBounds = null;
     _localTransformInverse = null;
     _projectedClip = null;
@@ -564,7 +564,7 @@ abstract class PersistedSurface implements ui.EngineLayer {
   /// Prints this surface into a [buffer] in a human-readable format.
   void debugPrint(StringBuffer buffer, int indent) {
     if (rootElement != null) {
-      buffer.write('${'  ' * indent}<${rootElement.tagName.toLowerCase()} ');
+      buffer.write('${'  ' * indent}<${rootElement!.tagName.toLowerCase()} ');
     } else {
       buffer.write('${'  ' * indent}<$runtimeType recycled ');
     }
@@ -572,7 +572,7 @@ abstract class PersistedSurface implements ui.EngineLayer {
     buffer.writeln('>');
     debugPrintChildren(buffer, indent);
     if (rootElement != null) {
-      buffer.writeln('${'  ' * indent}</${rootElement.tagName.toLowerCase()}>');
+      buffer.writeln('${'  ' * indent}</${rootElement!.tagName.toLowerCase()}>');
     } else {
       buffer.writeln('${'  ' * indent}</$runtimeType>');
     }
@@ -582,7 +582,7 @@ abstract class PersistedSurface implements ui.EngineLayer {
   @mustCallSuper
   void debugPrintAttributes(StringBuffer buffer) {
     if (rootElement != null) {
-      buffer.write('@${rootElement.hashCode} ');
+      buffer.write('@${rootElement!.hashCode} ');
     }
   }
 
@@ -618,7 +618,7 @@ abstract class PersistedContainerSurface extends PersistedSurface {
   ///
   /// `oldLayer` points to the surface rendered in the previous frame that's
   /// being updated by this layer.
-  PersistedContainerSurface(PersistedSurface oldLayer) : super(oldLayer) {
+  PersistedContainerSurface(PersistedSurface? oldLayer) : super(oldLayer) {
     assert(oldLayer == null || runtimeType == oldLayer.runtimeType);
   }
 
@@ -651,7 +651,7 @@ abstract class PersistedContainerSurface extends PersistedSurface {
 
   @override
   void recomputeTransformAndClip() {
-    _transform = parent._transform;
+    _transform = parent!._transform;
     _localClipBounds = null;
     _localTransformInverse = null;
     _projectedClip = null;
@@ -663,32 +663,32 @@ abstract class PersistedContainerSurface extends PersistedSurface {
     // Memoize length for efficiency.
     final int len = _children.length;
     // Memoize container element for efficiency. [childContainer] is polymorphic
-    final html.Element containerElement = childContainer;
+    final html.Element? containerElement = childContainer;
     for (int i = 0; i < len; i++) {
       final PersistedSurface child = _children[i];
       if (child.isPendingRetention) {
         assert(child.rootElement != null);
         child.retain();
       } else if (child is PersistedContainerSurface && child.oldLayer != null) {
-        final PersistedSurface oldLayer = child.oldLayer;
+        final PersistedSurface oldLayer = child.oldLayer!;
         assert(oldLayer.rootElement != null);
         assert(debugAssertSurfaceState(
             oldLayer, PersistedSurfaceState.pendingUpdate));
-        child.update(child.oldLayer);
+        child.update(child.oldLayer as PersistedContainerSurface);
       } else {
         assert(debugAssertSurfaceState(child, PersistedSurfaceState.created));
         assert(child.rootElement == null);
         child.build();
       }
-      containerElement.append(child.rootElement);
+      containerElement!.append(child.rootElement!);
       child._index = i;
     }
     _debugValidateContainerNewState();
   }
 
   @override
-  double matchForUpdate(PersistedContainerSurface existingSurface) {
-    assert(existingSurface.runtimeType == runtimeType);
+  double matchForUpdate(PersistedContainerSurface? existingSurface) {
+    assert(existingSurface!.runtimeType == runtimeType);
     // Intermediate container nodes don't have many resources worth comparing,
     // so we always return 1.0 to signal that it doesn't matter which one to
     // choose.
@@ -754,7 +754,7 @@ abstract class PersistedContainerSurface extends PersistedSurface {
         assert(debugAssertSurfaceState(newChild, PersistedSurfaceState.active,
             PersistedSurfaceState.pendingRetention));
         assert(newChild.rootElement != null);
-        assert(newChild.rootElement.parent == childContainer);
+        assert(newChild.rootElement!.parent == childContainer);
       }
       return true;
     }());
@@ -768,7 +768,7 @@ abstract class PersistedContainerSurface extends PersistedSurface {
     assert(oldSurface._children.isEmpty);
 
     // Memoizing variables for efficiency.
-    final html.Element containerElement = childContainer;
+    final html.Element? containerElement = childContainer;
     final int length = _children.length;
 
     for (int i = 0; i < length; i++) {
@@ -779,7 +779,7 @@ abstract class PersistedContainerSurface extends PersistedSurface {
             newChild, PersistedSurfaceState.pendingRetention));
       } else if (newChild is PersistedContainerSurface &&
           newChild.oldLayer != null) {
-        final PersistedContainerSurface oldLayer = newChild.oldLayer;
+        final PersistedContainerSurface oldLayer = newChild.oldLayer as PersistedContainerSurface;
         assert(debugAssertSurfaceState(
             oldLayer, PersistedSurfaceState.pendingUpdate));
         newChild.update(oldLayer);
@@ -792,7 +792,7 @@ abstract class PersistedContainerSurface extends PersistedSurface {
       }
       newChild._index = i;
       assert(newChild.rootElement != null);
-      containerElement.append(newChild.rootElement);
+      containerElement!.append(newChild.rootElement!);
     }
   }
 
@@ -823,8 +823,8 @@ abstract class PersistedContainerSurface extends PersistedSurface {
       assert(newChild.rootElement != null);
 
       // Move the HTML node if necessary.
-      if (newChild.rootElement.parent != childContainer) {
-        childContainer.append(newChild.rootElement);
+      if (newChild.rootElement!.parent != childContainer) {
+        childContainer!.append(newChild.rootElement!);
       }
 
       newChild.retain();
@@ -839,15 +839,15 @@ abstract class PersistedContainerSurface extends PersistedSurface {
     // are released.
     if (newChild is PersistedContainerSurface && newChild.oldLayer != null) {
       assert(debugAssertSurfaceState(
-          newChild.oldLayer, PersistedSurfaceState.pendingUpdate));
+          newChild.oldLayer!, PersistedSurfaceState.pendingUpdate));
       assert(newChild.rootElement == null);
-      assert(newChild.oldLayer.rootElement != null);
+      assert(newChild.oldLayer!.rootElement != null);
 
-      final PersistedContainerSurface oldLayer = newChild.oldLayer;
+      final PersistedContainerSurface oldLayer = newChild.oldLayer as PersistedContainerSurface;
 
       // Move the HTML node if necessary.
-      if (oldLayer.rootElement.parent != childContainer) {
-        childContainer.append(oldLayer.rootElement);
+      if (oldLayer.rootElement!.parent != childContainer) {
+        childContainer!.append(oldLayer.rootElement!);
       }
 
       newChild.update(oldLayer);
@@ -859,7 +859,7 @@ abstract class PersistedContainerSurface extends PersistedSurface {
 
     assert(debugAssertSurfaceState(newChild, PersistedSurfaceState.created));
 
-    PersistedSurface bestMatch;
+    PersistedSurface? bestMatch;
     double bestScore = 2.0;
     for (int i = 0; i < oldSurface._children.length; i++) {
       final PersistedSurface candidate = oldSurface._children[i];
@@ -878,15 +878,15 @@ abstract class PersistedContainerSurface extends PersistedSurface {
       newChild.update(bestMatch);
 
       // Move the HTML node if necessary.
-      if (newChild.rootElement.parent != childContainer) {
-        childContainer.append(newChild.rootElement);
+      if (newChild.rootElement!.parent != childContainer) {
+        childContainer!.append(newChild.rootElement!);
       }
 
       assert(
           debugAssertSurfaceState(bestMatch, PersistedSurfaceState.released));
     } else {
       newChild.build();
-      childContainer.append(newChild.rootElement);
+      childContainer!.append(newChild.rootElement!);
       assert(debugAssertSurfaceState(newChild, PersistedSurfaceState.active));
     }
 
@@ -907,15 +907,15 @@ abstract class PersistedContainerSurface extends PersistedSurface {
     assert(_children.isNotEmpty && oldSurface._children.isNotEmpty);
 
     // Memoize container element for efficiency. [childContainer] is polymorphic
-    final html.Element containerElement = childContainer;
-    final Map<PersistedSurface, PersistedSurface> matches =
+    final html.Element? containerElement = childContainer;
+    final Map<PersistedSurface?, PersistedSurface> matches =
         _matchChildren(oldSurface);
 
     // This pair of lists maps from _children indices to oldSurface._children indices.
     // These lists are initialized lazily, only when we discover that we will need to
     // move nodes around. Otherwise, these lists remain null.
-    List<int> indexMapNew;
-    List<int> indexMapOld;
+    List<int>? indexMapNew;
+    List<int>? indexMapOld;
 
     // Whether children need to move around the DOM. It is common for children
     // to be updated/retained but never move. Knowing this allows us to bypass
@@ -926,7 +926,7 @@ abstract class PersistedContainerSurface extends PersistedSurface {
       final PersistedSurface newChild = _children[topInNew];
 
       // The old child surface that `newChild` was updated or retained from.
-      PersistedSurface matchedOldChild;
+      PersistedSurface? matchedOldChild;
       // Whether the child is getting a new parent. This happens in the
       // following situations:
       // - It's a new child and is being attached for the first time.
@@ -935,15 +935,15 @@ abstract class PersistedContainerSurface extends PersistedSurface {
       bool isReparenting = true;
 
       if (newChild.isPendingRetention) {
-        isReparenting = newChild.rootElement.parent != containerElement;
+        isReparenting = newChild.rootElement!.parent != containerElement;
         newChild.retain();
         matchedOldChild = newChild;
         assert(debugAssertSurfaceState(
             newChild, PersistedSurfaceState.pendingRetention));
       } else if (newChild is PersistedContainerSurface &&
           newChild.oldLayer != null) {
-        final PersistedContainerSurface oldLayer = newChild.oldLayer;
-        isReparenting = oldLayer.rootElement.parent != containerElement;
+        final PersistedContainerSurface oldLayer = newChild.oldLayer as PersistedContainerSurface;
+        isReparenting = oldLayer.rootElement!.parent != containerElement;
         matchedOldChild = oldLayer;
         assert(debugAssertSurfaceState(
             oldLayer, PersistedSurfaceState.pendingUpdate));
@@ -956,7 +956,7 @@ abstract class PersistedContainerSurface extends PersistedSurface {
         if (matchedOldChild != null) {
           assert(debugAssertSurfaceState(
               matchedOldChild, PersistedSurfaceState.active));
-          isReparenting = matchedOldChild.rootElement.parent != containerElement;
+          isReparenting = matchedOldChild.rootElement!.parent != containerElement;
           newChild.update(matchedOldChild);
           assert(debugAssertSurfaceState(
               matchedOldChild, PersistedSurfaceState.released));
@@ -995,8 +995,8 @@ abstract class PersistedContainerSurface extends PersistedSurface {
         }
       }
       if (requiresDomInserts && indexInOld != -1) {
-        indexMapNew.add(topInNew);
-        indexMapOld.add(indexInOld);
+        indexMapNew!.add(topInNew);
+        indexMapOld!.add(indexInOld);
       }
 
       newChild._index = topInNew;
@@ -1008,8 +1008,8 @@ abstract class PersistedContainerSurface extends PersistedSurface {
     // Avoid calling `_insertChildDomNodes` unnecessarily. Only call it if we
     // actually need to move DOM nodes around.
     if (requiresDomInserts) {
-      assert(indexMapNew.length == indexMapOld.length);
-      _insertChildDomNodes(indexMapNew, indexMapOld);
+      assert(indexMapNew!.length == indexMapOld!.length);
+      _insertChildDomNodes(indexMapNew, indexMapOld!);
     } else {
       // The fast path, where nothing needs to move, should not intialize the
       // mapping lists at all.
@@ -1023,35 +1023,35 @@ abstract class PersistedContainerSurface extends PersistedSurface {
 
   /// Performs the minimum number of DOM moves necessary to put all children in
   /// the right place in the DOM.
-  void _insertChildDomNodes(List<int> indexMapNew, List<int> indexMapOld) {
-    final List<int> stationaryIndices = longestIncreasingSubsequence(indexMapOld);
+  void _insertChildDomNodes(List<int>? indexMapNew, List<int> indexMapOld) {
+    final List<int?> stationaryIndices = longestIncreasingSubsequence(indexMapOld);
 
     // Convert to stationary new indices
     for (int i = 0; i < stationaryIndices.length; i++) {
-      stationaryIndices[i] = indexMapNew[stationaryIndices[i]];
+      stationaryIndices[i] = indexMapNew![stationaryIndices[i]!];
     }
 
-    html.HtmlElement refNode;
-    final html.Element containerElement = childContainer;
+    html.HtmlElement? refNode;
+    final html.Element? containerElement = childContainer;
     for (int i = _children.length - 1; i >= 0; i -= 1) {
-      final int indexInNew = indexMapNew.indexOf(i);
+      final int indexInNew = indexMapNew!.indexOf(i);
       final bool isStationary = indexInNew != -1 && stationaryIndices.contains(i);
       final PersistedSurface child = _children[i];
-      final html.HtmlElement childElement = child.rootElement;
-      assert(childElement != null);
+      final html.HtmlElement childElement = child.rootElement as html.HtmlElement;
+      assert(childElement != null); // ignore: unnecessary_null_comparison
       if (!isStationary) {
         if (refNode == null) {
-          containerElement.append(childElement);
+          containerElement!.append(childElement);
         } else {
-          containerElement.insertBefore(childElement, refNode);
+          containerElement!.insertBefore(childElement, refNode);
         }
       }
       refNode = childElement;
-      assert(child.rootElement.parent == childContainer);
+      assert(child.rootElement!.parent == childContainer);
     }
   }
 
-  Map<PersistedSurface, PersistedSurface> _matchChildren(
+  Map<PersistedSurface?, PersistedSurface> _matchChildren(
       PersistedContainerSurface oldSurface) {
     final int newUnfilteredChildCount = _children.length;
     final int oldUnfilteredChildCount = oldSurface._children.length;
@@ -1068,7 +1068,7 @@ abstract class PersistedContainerSurface extends PersistedSurface {
     }
 
     // Extract old nodes that can be matched against.
-    final List<PersistedSurface> oldChildren = <PersistedSurface>[];
+    final List<PersistedSurface?> oldChildren = <PersistedSurface?>[];
     for (int i = 0; i < oldUnfilteredChildCount; i++) {
       final PersistedSurface child = oldSurface._children[i];
       if (child.isActive) {
@@ -1095,9 +1095,9 @@ abstract class PersistedContainerSurface extends PersistedSurface {
     for (int indexInNew = 0; indexInNew < newChildCount; indexInNew += 1) {
       final PersistedSurface newChild = newChildren[indexInNew];
       for (int indexInOld = 0; indexInOld < oldChildCount; indexInOld += 1) {
-        final PersistedSurface oldChild = oldChildren[indexInOld];
+        final PersistedSurface? oldChild = oldChildren[indexInOld];
         final bool childAlreadyClaimed = oldChild == null;
-        if (childAlreadyClaimed || !newChild.canUpdateAsMatch(oldChild)) {
+        if (childAlreadyClaimed || !newChild.canUpdateAsMatch(oldChild!)) {
           continue;
         }
         allMatches.add(_PersistedSurfaceMatch(
@@ -1109,19 +1109,19 @@ abstract class PersistedContainerSurface extends PersistedSurface {
     }
 
     allMatches.sort((_PersistedSurfaceMatch m1, _PersistedSurfaceMatch m2) {
-      return m1.matchQuality.compareTo(m2.matchQuality);
+      return m1.matchQuality!.compareTo(m2.matchQuality!);
     });
 
-    final Map<PersistedSurface, PersistedSurface> result =
-        <PersistedSurface, PersistedSurface>{};
+    final Map<PersistedSurface?, PersistedSurface> result =
+        <PersistedSurface?, PersistedSurface>{};
     for (int i = 0; i < allMatches.length; i += 1) {
       final _PersistedSurfaceMatch match = allMatches[i];
       // This may be null if it has been claimed.
-      final PersistedSurface matchedChild = oldChildren[match.oldChildIndex];
+      final PersistedSurface? matchedChild = oldChildren[match.oldChildIndex!];
       // Whether the new child hasn't found a match yet.
       final bool newChildNeedsMatch = result[match.newChild] == null;
       if (matchedChild != null && newChildNeedsMatch) {
-        oldChildren[match.oldChildIndex] = null;
+        oldChildren[match.oldChildIndex!] = null;
         result[match.newChild] = matchedChild; // claim it
       }
     }
@@ -1190,23 +1190,23 @@ class _PersistedSurfaceMatch {
 
   /// The child in the new scene who we are trying to match to an existing
   /// child.
-  final PersistedSurface newChild;
+  final PersistedSurface? newChild;
 
   /// The index pointing at the old child that matched [newChild] in the old
   /// child list.
   ///
   /// The indirection is intentional because when matches are traversed old
   /// children are claimed and nulled out in the array.
-  final int oldChildIndex;
+  final int? oldChildIndex;
 
   /// The score of how well [newChild] matched the old child as computed by
   /// [PersistedSurface.matchForUpdate].
-  final double matchQuality;
+  final double? matchQuality;
 
   @override
   String toString() {
     if (assertionsEnabled) {
-      return '_PersistedSurfaceMatch(${newChild.runtimeType}#${newChild.hashCode}: $oldChildIndex, quality: $matchQuality)';
+      return '_PersistedSurfaceMatch(${newChild!.runtimeType}#${newChild!.hashCode}: $oldChildIndex, quality: $matchQuality)';
     } else {
       return super.toString();
     }
diff --git a/lib/web_ui/lib/src/engine/surface/surface_stats.dart b/lib/web_ui/lib/src/engine/surface/surface_stats.dart
index 9bb6cab18a3b4..cb539752195d8 100644
--- a/lib/web_ui/lib/src/engine/surface/surface_stats.dart
+++ b/lib/web_ui/lib/src/engine/surface/surface_stats.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
 /// Surfaces that were retained this frame.
@@ -39,7 +38,7 @@ class _DebugSurfaceStats {
   _DebugSurfaceStats(this.surface);
 
   /// The surface these stats are for, or `null` if these are aggregated stats.
-  final PersistedSurface surface;
+  final PersistedSurface? surface;
 
   /// How many times a surface was retained from a previously rendered frame.
   int retainSurfaceCount = 0;
@@ -88,7 +87,7 @@ class _DebugSurfaceStats {
   }
 }
 
-html.CanvasRenderingContext2D _debugSurfaceStatsOverlayCtx;
+html.CanvasRenderingContext2D? _debugSurfaceStatsOverlayCtx;
 
 void _debugRepaintSurfaceStatsOverlay(PersistedScene scene) {
   final int overlayWidth = html.window.innerWidth;
@@ -115,10 +114,10 @@ void _debugRepaintSurfaceStatsOverlay(PersistedScene scene) {
       ..zIndex = '1000'
       ..opacity = '0.8';
     _debugSurfaceStatsOverlayCtx = _debugSurfaceStatsOverlay.context2D;
-    html.document.body.append(_debugSurfaceStatsOverlay);
+    html.document.body!.append(_debugSurfaceStatsOverlay);
   }
 
-  _debugSurfaceStatsOverlayCtx
+  _debugSurfaceStatsOverlayCtx!
     ..fillStyle = 'black'
     ..beginPath()
     ..rect(0, 0, overlayWidth, overlayHeight)
@@ -131,7 +130,7 @@ void _debugRepaintSurfaceStatsOverlay(PersistedScene scene) {
   final double physicsScreenPixelCount =
       physicalScreenWidth * physicalScreenHeight;
 
-  final int totalDomNodeCount = scene.rootElement.querySelectorAll('*').length;
+  final int totalDomNodeCount = scene.rootElement!.querySelectorAll('*').length;
 
   for (int i = 0; i < _surfaceStatsTimeline.length; i++) {
     final Map<PersistedSurface, _DebugSurfaceStats> statsMap =
@@ -141,7 +140,7 @@ void _debugRepaintSurfaceStatsOverlay(PersistedScene scene) {
     for (_DebugSurfaceStats oneSurfaceStats in statsMap.values) {
       totals.aggregate(oneSurfaceStats);
       if (oneSurfaceStats.surface is PersistedPicture) {
-        final PersistedPicture picture = oneSurfaceStats.surface;
+        final PersistedPicture picture = oneSurfaceStats.surface as PersistedPicture;
         pixelCount += picture.bitmapPixelCount;
       }
     }
@@ -155,7 +154,7 @@ void _debugRepaintSurfaceStatsOverlay(PersistedScene scene) {
         totals.retainSurfaceCount / _surfaceStatsTimeline[i].length;
 
     // Repaints
-    _debugSurfaceStatsOverlayCtx
+    _debugSurfaceStatsOverlayCtx!
       ..lineWidth = strokeWidth
       ..strokeStyle = 'red'
       ..beginPath()
@@ -164,7 +163,7 @@ void _debugRepaintSurfaceStatsOverlay(PersistedScene scene) {
       ..stroke();
 
     // DOM allocations
-    _debugSurfaceStatsOverlayCtx
+    _debugSurfaceStatsOverlayCtx!
       ..lineWidth = strokeWidth
       ..strokeStyle = 'red'
       ..beginPath()
@@ -173,7 +172,7 @@ void _debugRepaintSurfaceStatsOverlay(PersistedScene scene) {
       ..stroke();
 
     // Bitmap allocations
-    _debugSurfaceStatsOverlayCtx
+    _debugSurfaceStatsOverlayCtx!
       ..lineWidth = strokeWidth
       ..strokeStyle = 'red'
       ..beginPath()
@@ -182,7 +181,7 @@ void _debugRepaintSurfaceStatsOverlay(PersistedScene scene) {
       ..stroke();
 
     // Surface retentions
-    _debugSurfaceStatsOverlayCtx
+    _debugSurfaceStatsOverlayCtx!
       ..lineWidth = strokeWidth
       ..strokeStyle = 'green'
       ..beginPath()
@@ -191,7 +190,7 @@ void _debugRepaintSurfaceStatsOverlay(PersistedScene scene) {
       ..stroke();
   }
 
-  _debugSurfaceStatsOverlayCtx
+  _debugSurfaceStatsOverlayCtx!
     ..font = 'normal normal 14px sans-serif'
     ..fillStyle = 'white'
     ..fillText('Repaint rate', 5, rowHeight - 5)
@@ -200,7 +199,7 @@ void _debugRepaintSurfaceStatsOverlay(PersistedScene scene) {
     ..fillText('Retain rate', 5, 4 * rowHeight - 5);
 
   for (int i = 1; i <= rowCount; i++) {
-    _debugSurfaceStatsOverlayCtx
+    _debugSurfaceStatsOverlayCtx!
       ..lineWidth = 1
       ..strokeStyle = 'blue'
       ..beginPath()
@@ -231,7 +230,7 @@ void _debugPrintSurfaceStats(PersistedScene scene, int frameNumber) {
 
   void countReusesRecursively(PersistedSurface surface) {
     final _DebugSurfaceStats stats = _surfaceStatsFor(surface);
-    assert(stats != null);
+    assert(stats != null); // ignore: unnecessary_null_comparison
 
     surfaceRetainCount += stats.retainSurfaceCount;
     elementReuseCount += stats.reuseElementCount;
diff --git a/lib/web_ui/lib/src/engine/surface/transform.dart b/lib/web_ui/lib/src/engine/surface/transform.dart
index 470c4a8b74725..716090bc2e545 100644
--- a/lib/web_ui/lib/src/engine/surface/transform.dart
+++ b/lib/web_ui/lib/src/engine/surface/transform.dart
@@ -2,26 +2,25 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
 /// A surface that transforms its children using CSS transform.
 class PersistedTransform extends PersistedContainerSurface
     implements ui.TransformEngineLayer {
-  PersistedTransform(PersistedTransform oldLayer, this.matrix4)
+  PersistedTransform(PersistedTransform? oldLayer, this.matrix4)
       : super(oldLayer);
 
   final Float32List matrix4;
 
   @override
   void recomputeTransformAndClip() {
-    _transform = parent._transform.multiplied(Matrix4.fromFloat32List(matrix4));
+    _transform = parent!._transform!.multiplied(Matrix4.fromFloat32List(matrix4));
     _localTransformInverse = null;
     _projectedClip = null;
   }
 
   @override
-  Matrix4 get localTransformInverse {
+  Matrix4? get localTransformInverse {
     _localTransformInverse ??=
         Matrix4.tryInvert(Matrix4.fromFloat32List(matrix4));
     return _localTransformInverse;
@@ -35,7 +34,7 @@ class PersistedTransform extends PersistedContainerSurface
 
   @override
   void apply() {
-    rootElement.style.transform = float64ListToCssTransform(matrix4);
+    rootElement!.style.transform = float64ListToCssTransform(matrix4);
   }
 
   @override
diff --git a/lib/web_ui/lib/src/engine/test_embedding.dart b/lib/web_ui/lib/src/engine/test_embedding.dart
index c1295a2ff0921..9f55594399530 100644
--- a/lib/web_ui/lib/src/engine/test_embedding.dart
+++ b/lib/web_ui/lib/src/engine/test_embedding.dart
@@ -2,14 +2,14 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 const bool _debugLogHistoryActions = false;
 
 class TestHistoryEntry {
   final dynamic state;
-  final String title;
+  final String? title;
   final String url;
 
   const TestHistoryEntry(this.state, this.title, this.url);
@@ -77,7 +77,7 @@ class TestLocationStrategy extends LocationStrategy {
   }
 
   @override
-  void replaceState(dynamic state, String title, String url) {
+  void replaceState(dynamic state, String title, String? url) {
     assert(withinAppHistory);
     if (url == null || url == '') {
       url = currentEntry.url;
@@ -157,10 +157,10 @@ class TestLocationStrategy extends LocationStrategy {
 
   @override
   String toString() {
-    final List<String> lines = List<String>(history.length);
+    final List<String> lines = <String>[];
     for (int i = 0; i < history.length; i++) {
       final TestHistoryEntry entry = history[i];
-      lines[i] = _currentEntryIndex == i ? '* $entry' : '  $entry';
+      lines.add(_currentEntryIndex == i ? '* $entry' : '  $entry');
     }
     return '$runtimeType: [\n${lines.join('\n')}\n]';
   }
diff --git a/lib/web_ui/lib/src/engine/text/font_collection.dart b/lib/web_ui/lib/src/engine/text/font_collection.dart
index e68880fda8efc..91b1909964f4c 100644
--- a/lib/web_ui/lib/src/engine/text/font_collection.dart
+++ b/lib/web_ui/lib/src/engine/text/font_collection.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 const String _ahemFontFamily = 'Ahem';
@@ -17,8 +17,8 @@ const String _robotoFontUrl = 'packages/ui/assets/Roboto-Regular.ttf';
 /// font manifest. If test fonts are enabled, then call
 /// [registerTestFonts] as well.
 class FontCollection {
-  FontManager _assetFontManager;
-  FontManager _testFontManager;
+  FontManager? _assetFontManager;
+  FontManager? _testFontManager;
 
   /// Reads the font manifest using the [assetManager] and registers all of the
   /// fonts declared within.
@@ -37,12 +37,7 @@ class FontCollection {
       }
     }
 
-    if (byteData == null) {
-      throw AssertionError(
-          'There was a problem trying to load FontManifest.json');
-    }
-
-    final List<dynamic> fontManifest =
+    final List<dynamic>? fontManifest =
         json.decode(utf8.decode(byteData.buffer.asUint8List()));
     if (fontManifest == null) {
       throw AssertionError(
@@ -55,8 +50,8 @@ class FontCollection {
       _assetFontManager = _PolyfillFontManager();
     }
 
-    for (Map<String, dynamic> fontFamily in fontManifest) {
-      final String family = fontFamily['family'];
+    for (Map<String, dynamic> fontFamily in fontManifest.cast<Map<String, dynamic>>()) {
+      final String? family = fontFamily['family'];
       final List<dynamic> fontAssets = fontFamily['fonts'];
 
       for (dynamic fontAssetItem in fontAssets) {
@@ -68,22 +63,22 @@ class FontCollection {
             descriptors[descriptor] = '${fontAsset[descriptor]}';
           }
         }
-        _assetFontManager.registerAsset(
-            family, 'url(${assetManager.getAssetUrl(asset)})', descriptors);
+        _assetFontManager!.registerAsset(
+            family!, 'url(${assetManager.getAssetUrl(asset)})', descriptors);
       }
     }
   }
 
-  Future<void> loadFontFromList(Uint8List list, {String fontFamily}) {
-    return _assetFontManager._loadFontFaceBytes(fontFamily, list);
+  Future<void> loadFontFromList(Uint8List list, {required String fontFamily}) {
+    return _assetFontManager!._loadFontFaceBytes(fontFamily, list);
   }
 
   /// Registers fonts that are used by tests.
   void debugRegisterTestFonts() {
     _testFontManager = FontManager();
-    _testFontManager.registerAsset(
+    _testFontManager!.registerAsset(
         _ahemFontFamily, 'url($_ahemFontUrl)', const <String, String>{});
-    _testFontManager.registerAsset(
+    _testFontManager!.registerAsset(
         _robotoFontFamily, 'url($_robotoFontUrl)', const <String, String>{});
   }
 
@@ -256,14 +251,14 @@ class _PolyfillFontManager extends FontManager {
     }
     paragraph.text = _testString;
 
-    html.document.body.append(paragraph);
+    html.document.body!.append(paragraph);
     final int sansSerifWidth = paragraph.offsetWidth;
 
     paragraph.style.fontFamily = "'$family', $fallbackFontName";
 
     final Completer<void> completer = Completer<void>();
 
-    DateTime _fontLoadStart;
+    late DateTime _fontLoadStart;
 
     void _watchWidth() {
       if (paragraph.offsetWidth != sansSerifWidth) {
@@ -281,7 +276,7 @@ class _PolyfillFontManager extends FontManager {
       }
     }
 
-    final Map<String, String> fontStyleMap = <String, String>{};
+    final Map<String, String?> fontStyleMap = <String, String?>{};
     fontStyleMap['font-family'] = "'$family'";
     fontStyleMap['src'] = asset;
     if (descriptors['style'] != null) {
@@ -296,7 +291,7 @@ class _PolyfillFontManager extends FontManager {
     final html.StyleElement fontLoadStyle = html.StyleElement();
     fontLoadStyle.type = 'text/css';
     fontLoadStyle.innerHtml = '@font-face { $fontFaceDeclaration }';
-    html.document.head.append(fontLoadStyle);
+    html.document.head!.append(fontLoadStyle);
 
     // HACK: If this is an icon font, then when it loads it won't change the
     // width of our test string. So we just have to hope it loads before the
@@ -314,4 +309,4 @@ class _PolyfillFontManager extends FontManager {
 }
 
 final bool supportsFontLoadingApi = js_util.hasProperty(html.window, 'FontFace');
-final bool supportsFontsClearApi = html.document.fonts != null && js_util.hasProperty(html.document.fonts, 'clear');
+final bool supportsFontsClearApi = js_util.hasProperty(html.document, 'fonts') && js_util.hasProperty(html.document.fonts, 'clear');
diff --git a/lib/web_ui/lib/src/engine/text/line_break_properties.dart b/lib/web_ui/lib/src/engine/text/line_break_properties.dart
index 291736d2438d6..d475a3bcb9961 100644
--- a/lib/web_ui/lib/src/engine/text/line_break_properties.dart
+++ b/lib/web_ui/lib/src/engine/text/line_break_properties.dart
@@ -12,7 +12,7 @@
 // # Unicode and the Unicode Logo are registered trademarks of Unicode, Inc. in the U.S. and other countries.
 // # For terms of use, see http://www.unicode.org/terms_of_use.html
 
-// @dart = 2.6
+
 part of engine;
 
 /// For an explanation of these enum values, see:
diff --git a/lib/web_ui/lib/src/engine/text/line_breaker.dart b/lib/web_ui/lib/src/engine/text/line_breaker.dart
index dff40c10a7764..eac5430ae5419 100644
--- a/lib/web_ui/lib/src/engine/text/line_breaker.dart
+++ b/lib/web_ui/lib/src/engine/text/line_breaker.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Various types of line breaks as defined by the Unicode spec.
@@ -29,7 +29,7 @@ class LineBreakResult {
 }
 
 /// Normalizes properties that behave the same way into one common property.
-LineCharProperty _normalizeLineProperty(LineCharProperty prop) {
+LineCharProperty? _normalizeLineProperty(LineCharProperty? prop) {
   // LF and NL behave exactly the same as BK.
   // See: https://www.unicode.org/reports/tr14/tr14-22.html#ExplicitBreaks
   if (prop == LineCharProperty.LF || prop == LineCharProperty.NL) {
@@ -56,11 +56,11 @@ LineCharProperty _normalizeLineProperty(LineCharProperty prop) {
 /// * https://www.unicode.org/Public/11.0.0/ucd/LineBreak.txt
 LineBreakResult nextLineBreak(String text, int index) {
   // "raw" refers to the property before normalization.
-  LineCharProperty rawCurr = lineLookup.find(text, index);
-  LineCharProperty curr = _normalizeLineProperty(rawCurr);
+  LineCharProperty? rawCurr = lineLookup.find(text, index);
+  LineCharProperty? curr = _normalizeLineProperty(rawCurr);
 
-  LineCharProperty rawPrev;
-  LineCharProperty prev;
+  LineCharProperty? rawPrev;
+  LineCharProperty? prev;
 
   bool hasSpaces = false;
 
diff --git a/lib/web_ui/lib/src/engine/text/measurement.dart b/lib/web_ui/lib/src/engine/text/measurement.dart
index eb625144d72da..7d844f913418a 100644
--- a/lib/web_ui/lib/src/engine/text/measurement.dart
+++ b/lib/web_ui/lib/src/engine/text/measurement.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 // TODO(yjbanov): this is a hack we use to compute ideographic baseline; this
@@ -17,7 +17,7 @@ const double _baselineRatioHack = 1.1662499904632568;
 typedef CharPredicate = bool Function(int char);
 
 bool _whitespacePredicate(int char) {
-  final LineCharProperty prop =
+  final LineCharProperty? prop =
       _normalizeLineProperty(lineLookup.findForChar(char));
   return prop == LineCharProperty.SP ||
       prop == LineCharProperty.BK ||
@@ -25,7 +25,7 @@ bool _whitespacePredicate(int char) {
 }
 
 bool _newlinePredicate(int char) {
-  final LineCharProperty prop =
+  final LineCharProperty? prop =
       _normalizeLineProperty(lineLookup.findForChar(char));
   return prop == LineCharProperty.BK || prop == LineCharProperty.CR;
 }
@@ -35,7 +35,7 @@ bool _newlinePredicate(int char) {
 ///
 /// All instances of [ParagraphRuler] should be created through this class.
 class RulerManager {
-  RulerManager({@required this.rulerCacheCapacity}) {
+  RulerManager({required this.rulerCacheCapacity}) {
     _rulerHost.style
       ..position = 'fixed'
       ..visibility = 'hidden'
@@ -44,7 +44,7 @@ class RulerManager {
       ..left = '0'
       ..width = '0'
       ..height = '0';
-    html.document.body.append(_rulerHost);
+    html.document.body!.append(_rulerHost);
     registerHotRestartListener(dispose);
   }
 
@@ -89,7 +89,7 @@ class RulerManager {
   ///
   /// After this is called, this object is no longer usable.
   void dispose() {
-    _rulerHost?.remove();
+    _rulerHost.remove();
   }
 
   // Evicts all rulers from the cache.
@@ -148,7 +148,7 @@ class RulerManager {
   /// elsewhere.
   @visibleForTesting
   ParagraphRuler findOrCreateRuler(ParagraphGeometricStyle style) {
-    ParagraphRuler ruler = _rulers[style];
+    ParagraphRuler? ruler = _rulers[style];
     if (ruler == null) {
       if (assertionsEnabled) {
         domRenderer.debugRulerCacheMiss();
@@ -177,14 +177,14 @@ abstract class TextMeasurementService {
 
   /// Initializes the text measurement service with a specific
   /// [rulerCacheCapacity] that gets passed to the [RulerManager].
-  static void initialize({@required int rulerCacheCapacity}) {
+  static void initialize({required int rulerCacheCapacity}) {
     rulerManager?.dispose();
     rulerManager = null;
     rulerManager = RulerManager(rulerCacheCapacity: rulerCacheCapacity);
   }
 
   @visibleForTesting
-  static RulerManager rulerManager;
+  static RulerManager? rulerManager;
 
   /// The DOM-based text measurement service.
   @visibleForTesting
@@ -208,8 +208,8 @@ abstract class TextMeasurementService {
     // Skip using canvas measurements until the iframe becomes visible.
     // see: https://github.com/flutter/flutter/issues/36341
     if (!window.physicalSize.isEmpty &&
-        WebExperiments.instance.useCanvasText &&
-        _canUseCanvasMeasurement(paragraph)) {
+        WebExperiments.instance!.useCanvasText &&
+        _canUseCanvasMeasurement(paragraph as EngineParagraph)) {
       return canvasInstance;
     }
     return domInstance;
@@ -233,14 +233,14 @@ abstract class TextMeasurementService {
   }
 
   /// Measures the paragraph and returns a [MeasurementResult] object.
-  MeasurementResult measure(
+  MeasurementResult? measure(
     EngineParagraph paragraph,
     ui.ParagraphConstraints constraints,
   ) {
     assert(rulerManager != null);
     final ParagraphGeometricStyle style = paragraph._geometricStyle;
     final ParagraphRuler ruler =
-        TextMeasurementService.rulerManager.findOrCreateRuler(style);
+        TextMeasurementService.rulerManager!.findOrCreateRuler(style);
 
     if (assertionsEnabled) {
       if (paragraph._plainText == null) {
@@ -250,7 +250,7 @@ abstract class TextMeasurementService {
       }
     }
 
-    MeasurementResult result = ruler.cacheLookup(paragraph, constraints);
+    MeasurementResult? result = ruler.cacheLookup(paragraph, constraints);
     if (result != null) {
       return result;
     }
@@ -266,24 +266,24 @@ abstract class TextMeasurementService {
 
   /// Returns text position given a paragraph, constraints and offset.
   ui.TextPosition getTextPositionForOffset(EngineParagraph paragraph,
-      ui.ParagraphConstraints constraints, ui.Offset offset);
+      ui.ParagraphConstraints? constraints, ui.Offset offset);
 
   /// Delegates to a [ParagraphRuler] to measure a list of text boxes that
   /// enclose the given range of text.
   List<ui.TextBox> measureBoxesForRange(
     EngineParagraph paragraph,
     ui.ParagraphConstraints constraints, {
-    int start,
-    int end,
-    double alignOffset,
-    ui.TextDirection textDirection,
+    required int start,
+    required int end,
+    required double alignOffset,
+    required ui.TextDirection textDirection,
   }) {
     final ParagraphGeometricStyle style = paragraph._geometricStyle;
     final ParagraphRuler ruler =
-        TextMeasurementService.rulerManager.findOrCreateRuler(style);
+        TextMeasurementService.rulerManager!.findOrCreateRuler(style);
 
     return ruler.measureBoxesForRange(
-      paragraph._plainText,
+      paragraph._plainText!,
       constraints,
       start: start,
       end: end,
@@ -328,7 +328,7 @@ class DomTextMeasurementService extends TextMeasurementService {
   static DomTextMeasurementService get instance =>
       _instance ??= DomTextMeasurementService();
 
-  static DomTextMeasurementService _instance;
+  static DomTextMeasurementService? _instance;
 
   @override
   MeasurementResult _doMeasure(
@@ -337,7 +337,7 @@ class DomTextMeasurementService extends TextMeasurementService {
     ParagraphRuler ruler,
   ) {
     ruler.willMeasure(paragraph);
-    final String plainText = paragraph._plainText;
+    final String? plainText = paragraph._plainText;
 
     ruler.measureAll(constraints);
 
@@ -361,12 +361,12 @@ class DomTextMeasurementService extends TextMeasurementService {
     assert(paragraph._plainText != null);
     final ParagraphGeometricStyle style = paragraph._geometricStyle;
     final ParagraphRuler ruler =
-        TextMeasurementService.rulerManager.findOrCreateRuler(style);
+        TextMeasurementService.rulerManager!.findOrCreateRuler(style);
 
-    final String text = paragraph._plainText.substring(start, end);
+    final String text = paragraph._plainText!.substring(start, end);
     final ui.Paragraph substringParagraph = paragraph._cloneWithText(text);
 
-    ruler.willMeasure(substringParagraph);
+    ruler.willMeasure(substringParagraph as EngineParagraph);
     ruler.measureAsSingleLine();
     final TextDimensions dimensions = ruler.singleLineDimensions;
     ruler.didMeasure();
@@ -375,17 +375,17 @@ class DomTextMeasurementService extends TextMeasurementService {
 
   @override
   ui.TextPosition getTextPositionForOffset(EngineParagraph paragraph,
-      ui.ParagraphConstraints constraints, ui.Offset offset) {
+      ui.ParagraphConstraints? constraints, ui.Offset offset) {
     assert(
-      paragraph._measurementResult.lines == null,
+      paragraph._measurementResult!.lines == null,
       'should only be called when the faster lines-based approach is not possible',
     );
 
     final ParagraphGeometricStyle style = paragraph._geometricStyle;
     final ParagraphRuler ruler =
-        TextMeasurementService.rulerManager.findOrCreateRuler(style);
+        TextMeasurementService.rulerManager!.findOrCreateRuler(style);
     ruler.willMeasure(paragraph);
-    final int position = ruler.hitTest(constraints, offset);
+    final int position = ruler.hitTest(constraints!, offset);
     ruler.didMeasure();
     return ui.TextPosition(offset: position);
   }
@@ -418,8 +418,8 @@ class DomTextMeasurementService extends TextMeasurementService {
         _applySubPixelRoundingHack(minIntrinsicWidth, maxIntrinsicWidth);
     final double ideographicBaseline = alphabeticBaseline * _baselineRatioHack;
 
-    final String text = paragraph._plainText;
-    List<EngineLineMetrics> lines;
+    final String? text = paragraph._plainText;
+    List<EngineLineMetrics>? lines;
     if (text != null) {
       final double lineWidth = maxIntrinsicWidth;
       final double alignOffset = _calculateAlignOffsetForLine(
@@ -478,13 +478,13 @@ class DomTextMeasurementService extends TextMeasurementService {
     final double naturalHeight = ruler.constrainedDimensions.height;
 
     double height;
-    double lineHeight;
-    final int maxLines = paragraph._geometricStyle.maxLines;
+    double? lineHeight;
+    final int? maxLines = paragraph._geometricStyle.maxLines;
     if (maxLines == null) {
       height = naturalHeight;
     } else {
       // Lazily compute [lineHeight] when [maxLines] is not null.
-      lineHeight = ruler.lineHeightDimensions.height;
+      lineHeight = ruler.lineHeightDimensions!.height;
       height = math.min(naturalHeight, maxLines * lineHeight);
     }
 
@@ -546,7 +546,7 @@ class CanvasTextMeasurementService extends TextMeasurementService {
   static CanvasTextMeasurementService get instance =>
       _instance ??= CanvasTextMeasurementService();
 
-  static CanvasTextMeasurementService _instance;
+  static CanvasTextMeasurementService? _instance;
 
   final html.CanvasRenderingContext2D _canvasContext =
       html.CanvasElement().context2D;
@@ -557,9 +557,9 @@ class CanvasTextMeasurementService extends TextMeasurementService {
     ui.ParagraphConstraints constraints,
     ParagraphRuler ruler,
   ) {
-    final String text = paragraph._plainText;
+    final String text = paragraph._plainText!;
     final ParagraphGeometricStyle style = paragraph._geometricStyle;
-    assert(text != null);
+    assert(text != null); // ignore: unnecessary_null_comparison
 
     // TODO(mdebbar): Check if the whole text can fit in a single-line. Then avoid all this ceremony.
     _canvasContext.font = style.cssFontString;
@@ -592,12 +592,12 @@ class CanvasTextMeasurementService extends TextMeasurementService {
     }
 
     final int lineCount = linesCalculator.lines.length;
-    final double lineHeight = ruler.lineHeightDimensions.height;
+    final double lineHeight = ruler.lineHeightDimensions!.height;
     final double naturalHeight = lineCount * lineHeight;
-
-    final double height = style.maxLines == null
+    final int? maxLines = style.maxLines;
+    final double height = maxLines == null
         ? naturalHeight
-        : math.min(lineCount, style.maxLines) * lineHeight;
+        : math.min<int>(lineCount, maxLines) * lineHeight;
 
     final MeasurementResult result = MeasurementResult(
       constraints.width,
@@ -624,7 +624,7 @@ class CanvasTextMeasurementService extends TextMeasurementService {
   @override
   double measureSubstringWidth(EngineParagraph paragraph, int start, int end) {
     assert(paragraph._plainText != null);
-    final String text = paragraph._plainText;
+    final String text = paragraph._plainText!;
     final ParagraphGeometricStyle style = paragraph._geometricStyle;
     _canvasContext.font = style.cssFontString;
     return _measureSubstring(
@@ -638,7 +638,7 @@ class CanvasTextMeasurementService extends TextMeasurementService {
 
   @override
   ui.TextPosition getTextPositionForOffset(EngineParagraph paragraph,
-      ui.ParagraphConstraints constraints, ui.Offset offset) {
+      ui.ParagraphConstraints? constraints, ui.Offset offset) {
     // TODO(flutter_web): implement.
     return const ui.TextPosition(offset: 0);
   }
@@ -651,7 +651,7 @@ class CanvasTextMeasurementService extends TextMeasurementService {
 int _lastStart = -1;
 int _lastEnd = -1;
 String _lastText = '';
-ParagraphGeometricStyle _lastStyle;
+ParagraphGeometricStyle? _lastStyle;
 double _lastWidth = -1;
 
 /// Measures the width of the substring of [text] starting from the index
@@ -690,7 +690,7 @@ double _measureSubstring(
   final String sub =
       start == 0 && end == text.length ? text : text.substring(start, end);
   final double width =
-      _canvasContext.measureText(sub).width + letterSpacing * sub.length;
+      _canvasContext.measureText(sub).width + letterSpacing * sub.length as double;
 
   // What we are doing here is we are rounding to the nearest 2nd decimal
   // point. So 39.999423 becomes 40, and 11.243982 becomes 11.24.
@@ -731,7 +731,7 @@ class LinesCalculator {
   final EngineParagraph _paragraph;
   final double _maxWidth;
 
-  String get _text => _paragraph._plainText;
+  String? get _text => _paragraph._plainText;
   ParagraphGeometricStyle get _style => _paragraph._geometricStyle;
 
   /// The lines that have been consumed so far.
@@ -741,9 +741,9 @@ class LinesCalculator {
   int _chunkStart = 0;
   bool _reachedMaxLines = false;
 
-  double _cachedEllipsisWidth;
+  double? _cachedEllipsisWidth;
   double get _ellipsisWidth => _cachedEllipsisWidth ??=
-      _roundWidth(_canvasContext.measureText(_style.ellipsis).width);
+      _roundWidth(_canvasContext.measureText(_style.ellipsis!).width as double);
 
   bool get hasEllipsis => _style.ellipsis != null;
   bool get unlimitedLines => _style.maxLines == null;
@@ -757,7 +757,7 @@ class LinesCalculator {
         brk.type == LineBreakType.endOfText;
     final int chunkEnd = brk.index;
     final int chunkEndWithoutSpace =
-        _excludeTrailing(_text, _chunkStart, chunkEnd, _whitespacePredicate);
+        _excludeTrailing(_text!, _chunkStart, chunkEnd, _whitespacePredicate);
 
     // A single chunk of text could be force-broken into multiple lines if it
     // doesn't fit in a single line. That's why we need a loop.
@@ -798,11 +798,11 @@ class LinesCalculator {
           maxWidth: _maxWidth,
         );
         lines.add(EngineLineMetrics.withText(
-          _text.substring(_lineStart, breakingPoint) + _style.ellipsis,
+          _text!.substring(_lineStart, breakingPoint) + _style.ellipsis!,
           startIndex: _lineStart,
           endIndex: chunkEnd,
           endIndexWithoutNewlines:
-              _excludeTrailing(_text, _chunkStart, chunkEnd, _newlinePredicate),
+              _excludeTrailing(_text!, _chunkStart, chunkEnd, _newlinePredicate),
           hardBreak: false,
           width: widthOfResultingLine,
           left: alignOffset,
@@ -842,17 +842,17 @@ class LinesCalculator {
   }
 
   void _addLineBreak({
-    @required int lineEnd,
-    @required bool isHardBreak,
+    required int lineEnd,
+    required bool isHardBreak,
   }) {
     final int endWithoutNewlines = _excludeTrailing(
-      _text,
+      _text!,
       _lineStart,
       lineEnd,
       _newlinePredicate,
     );
     final int endWithoutSpace = _excludeTrailing(
-      _text,
+      _text!,
       _lineStart,
       endWithoutNewlines,
       _whitespacePredicate,
@@ -865,7 +865,7 @@ class LinesCalculator {
       maxWidth: _maxWidth,
     );
     final EngineLineMetrics metrics = EngineLineMetrics.withText(
-      _text.substring(_lineStart, endWithoutNewlines),
+      _text!.substring(_lineStart, endWithoutNewlines),
       startIndex: _lineStart,
       endIndex: lineEnd,
       endIndexWithoutNewlines: endWithoutNewlines,
@@ -887,7 +887,7 @@ class LinesCalculator {
   /// This method uses [_text], [_style] and [_canvasContext] to perform the
   /// measurement.
   double measureSubstring(int start, int end) {
-    return _measureSubstring(_canvasContext, _style, _text, start, end);
+    return _measureSubstring(_canvasContext, _style, _text!, start, end);
   }
 
   /// In a continuous block of text, finds the point where text can be broken to
@@ -896,13 +896,13 @@ class LinesCalculator {
   /// This always returns at least one character even if there isn't enough
   /// space for it.
   int forceBreakSubstring({
-    @required double maxWidth,
-    @required int start,
-    @required int end,
+    required double maxWidth,
+    required int start,
+    required int end,
   }) {
     assert(0 <= start);
     assert(start < end);
-    assert(end <= _text.length);
+    assert(end <= _text!.length);
 
     // When there's no ellipsis, the breaking point should be at least one
     // character away from [start].
@@ -1001,9 +1001,9 @@ class MaxIntrinsicCalculator {
 
 /// Calculates the offset necessary for the given line to be correctly aligned.
 double _calculateAlignOffsetForLine({
-  @required EngineParagraph paragraph,
-  @required double lineWidth,
-  @required double maxWidth,
+  required EngineParagraph paragraph,
+  required double lineWidth,
+  required double maxWidth,
 }) {
   final double emptySpace = maxWidth - lineWidth;
   // WARNING: the [paragraph] may not be laid out yet at this point. This
diff --git a/lib/web_ui/lib/src/engine/text/paragraph.dart b/lib/web_ui/lib/src/engine/text/paragraph.dart
index c02a81c308032..4903f330eaa87 100644
--- a/lib/web_ui/lib/src/engine/text/paragraph.dart
+++ b/lib/web_ui/lib/src/engine/text/paragraph.dart
@@ -2,50 +2,49 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
 class EngineLineMetrics implements ui.LineMetrics {
   EngineLineMetrics({
-    this.hardBreak,
-    this.ascent,
-    this.descent,
-    this.unscaledAscent,
-    this.height,
-    this.width,
-    this.left,
-    this.baseline,
-    this.lineNumber,
+    required this.hardBreak,
+    required this.ascent,
+    required this.descent,
+    required this.unscaledAscent,
+    required this.height,
+    required this.width,
+    required this.left,
+    required this.baseline,
+    required this.lineNumber,
   })  : displayText = null,
         startIndex = -1,
         endIndex = -1,
         endIndexWithoutNewlines = -1;
 
   EngineLineMetrics.withText(
-    this.displayText, {
-    @required this.startIndex,
-    @required this.endIndex,
-    @required this.endIndexWithoutNewlines,
-    @required this.hardBreak,
-    this.ascent,
-    this.descent,
-    this.unscaledAscent,
-    this.height,
-    @required this.width,
-    @required this.left,
-    this.baseline,
-    @required this.lineNumber,
-  })  : assert(displayText != null),
-        assert(startIndex != null),
-        assert(endIndex != null),
-        assert(endIndexWithoutNewlines != null),
-        assert(hardBreak != null),
-        assert(width != null),
-        assert(left != null),
-        assert(lineNumber != null && lineNumber >= 0);
+    String this.displayText, {
+    required this.startIndex,
+    required this.endIndex,
+    required this.endIndexWithoutNewlines,
+    required this.hardBreak,
+    required this.width,
+    required this.left,
+    required this.lineNumber,
+  })  : assert(displayText != null), // ignore: unnecessary_null_comparison
+        assert(startIndex != null), // ignore: unnecessary_null_comparison
+        assert(endIndex != null), // ignore: unnecessary_null_comparison
+        assert(endIndexWithoutNewlines != null), // ignore: unnecessary_null_comparison
+        assert(hardBreak != null), // ignore: unnecessary_null_comparison
+        assert(width != null), // ignore: unnecessary_null_comparison
+        assert(left != null), // ignore: unnecessary_null_comparison
+        assert(lineNumber != null && lineNumber >= 0), // ignore: unnecessary_null_comparison
+        ascent = double.infinity,
+        descent = double.infinity,
+        unscaledAscent = double.infinity,
+        height = double.infinity,
+        baseline = double.infinity;
 
   /// The text to be rendered on the screen representing this line.
-  final String displayText;
+  final String? displayText;
 
   /// The index (inclusive) in the text where this line begins.
   final int startIndex;
@@ -126,6 +125,23 @@ class EngineLineMetrics implements ui.LineMetrics {
         baseline == typedOther.baseline &&
         lineNumber == typedOther.lineNumber;
   }
+
+  @override
+  String toString() {
+    if (assertionsEnabled) {
+      return 'LineMetrics(hardBreak: $hardBreak, '
+                        'ascent: $ascent, '
+                        'descent: $descent, '
+                        'unscaledAscent: $unscaledAscent, '
+                        'height: $height, '
+                        'width: $width, '
+                        'left: $left, '
+                        'baseline: $baseline, '
+                        'lineNumber: $lineNumber)';
+    } else {
+      return super.toString();
+    }
+  }
 }
 
 /// The web implementation of [ui.Paragraph].
@@ -135,13 +151,13 @@ class EngineParagraph implements ui.Paragraph {
   ///
   /// To create a [ui.Paragraph] object, use a [ui.ParagraphBuilder].
   EngineParagraph({
-    @required html.HtmlElement paragraphElement,
-    @required ParagraphGeometricStyle geometricStyle,
-    @required String plainText,
-    @required ui.Paint paint,
-    @required ui.TextAlign textAlign,
-    @required ui.TextDirection textDirection,
-    @required ui.Paint background,
+    required html.HtmlElement paragraphElement,
+    required ParagraphGeometricStyle geometricStyle,
+    required String? plainText,
+    required ui.Paint? paint,
+    required ui.TextAlign textAlign,
+    required ui.TextDirection textDirection,
+    required ui.Paint? background,
   })  : assert((plainText == null && paint == null) ||
             (plainText != null && paint != null)),
         _paragraphElement = paragraphElement,
@@ -149,19 +165,19 @@ class EngineParagraph implements ui.Paragraph {
         _plainText = plainText,
         _textAlign = textAlign,
         _textDirection = textDirection,
-        _paint = paint,
-        _background = background;
+        _paint = paint as SurfacePaint?,
+        _background = background as SurfacePaint?;
 
   final html.HtmlElement _paragraphElement;
   final ParagraphGeometricStyle _geometricStyle;
-  final String _plainText;
-  final SurfacePaint _paint;
+  final String? _plainText;
+  final SurfacePaint? _paint;
   final ui.TextAlign _textAlign;
   final ui.TextDirection _textDirection;
-  final SurfacePaint _background;
+  final SurfacePaint? _background;
 
   @visibleForTesting
-  String get plainText => _plainText;
+  String? get plainText => _plainText;
 
   @visibleForTesting
   html.HtmlElement get paragraphElement => _paragraphElement;
@@ -175,7 +191,7 @@ class EngineParagraph implements ui.Paragraph {
       TextMeasurementService.forParagraph(this);
 
   /// The measurement result of the last layout operation.
-  MeasurementResult _measurementResult;
+  MeasurementResult? _measurementResult;
 
   bool get _hasLineMetrics => _measurementResult?.lines != null;
 
@@ -202,7 +218,7 @@ class EngineParagraph implements ui.Paragraph {
   double get longestLine {
     if (_hasLineMetrics) {
       double maxWidth = 0.0;
-      for (ui.LineMetrics metrics in _measurementResult.lines) {
+      for (ui.LineMetrics metrics in _measurementResult!.lines!) {
         if (maxWidth < metrics.width) {
           maxWidth = metrics.width;
         }
@@ -232,7 +248,7 @@ class EngineParagraph implements ui.Paragraph {
   bool get didExceedMaxLines => _didExceedMaxLines;
   bool _didExceedMaxLines = false;
 
-  ui.ParagraphConstraints _lastUsedConstraints;
+  ui.ParagraphConstraints? _lastUsedConstraints;
 
   /// Returns horizontal alignment offset for single line text when rendering
   /// directly into a canvas without css text alignment styling.
@@ -255,14 +271,14 @@ class EngineParagraph implements ui.Paragraph {
       return;
     }
 
-    Stopwatch stopwatch;
+    late Stopwatch stopwatch;
     if (Profiler.isBenchmarkMode) {
       stopwatch = Stopwatch()..start();
     }
     _measurementResult = _measurementService.measure(this, constraints);
     if (Profiler.isBenchmarkMode) {
       stopwatch.stop();
-      Profiler.instance.benchmark('text_layout', stopwatch.elapsedMicroseconds);
+      Profiler.instance.benchmark('text_layout', stopwatch.elapsedMicroseconds.toDouble());
     }
 
     _lastUsedConstraints = constraints;
@@ -273,7 +289,7 @@ class EngineParagraph implements ui.Paragraph {
       _didExceedMaxLines = false;
     }
 
-    if (_measurementResult.isSingleLine && constraints != null) {
+    if (_measurementResult!.isSingleLine) {
       switch (_textAlign) {
         case ui.TextAlign.center:
           _alignOffset = (constraints.width - maxIntrinsicWidth) / 2.0;
@@ -357,8 +373,8 @@ class EngineParagraph implements ui.Paragraph {
     ui.BoxHeightStyle boxHeightStyle = ui.BoxHeightStyle.tight,
     ui.BoxWidthStyle boxWidthStyle = ui.BoxWidthStyle.tight,
   }) {
-    assert(boxHeightStyle != null);
-    assert(boxWidthStyle != null);
+    assert(boxHeightStyle != null); // ignore: unnecessary_null_comparison
+    assert(boxWidthStyle != null); // ignore: unnecessary_null_comparison
     // Zero-length ranges and invalid ranges return an empty list.
     if (start == end || start < 0 || end < 0) {
       return <ui.TextBox>[];
@@ -369,11 +385,11 @@ class EngineParagraph implements ui.Paragraph {
     // https://github.com/flutter/flutter/issues/55587
     if (_plainText == null) {
       return <ui.TextBox>[
-        ui.TextBox.fromLTRBD(0, 0, 0, _lineHeight, _textDirection)
+        ui.TextBox.fromLTRBD(0, 0, 0, _lineHeight, _textDirection),
       ];
     }
 
-    final int length = _plainText.length;
+    final int length = _plainText!.length;
     // Ranges that are out of bounds should return an empty list.
     if (start > length || end > length) {
       return <ui.TextBox>[];
@@ -384,7 +400,7 @@ class EngineParagraph implements ui.Paragraph {
     if (!_hasLineMetrics) {
       return _measurementService.measureBoxesForRange(
         this,
-        _lastUsedConstraints,
+        _lastUsedConstraints!,
         start: start,
         end: end,
         alignOffset: _alignOffset,
@@ -392,7 +408,7 @@ class EngineParagraph implements ui.Paragraph {
       );
     }
 
-    final List<EngineLineMetrics> lines = _measurementResult.lines;
+    final List<EngineLineMetrics> lines = _measurementResult!.lines!;
     if (start >= lines.last.endIndex) {
       return <ui.TextBox>[];
     }
@@ -444,7 +460,7 @@ class EngineParagraph implements ui.Paragraph {
   ui.Paragraph _cloneWithText(String plainText) {
     return EngineParagraph(
       plainText: plainText,
-      paragraphElement: _paragraphElement.clone(true),
+      paragraphElement: _paragraphElement.clone(true) as html.HtmlElement,
       geometricStyle: _geometricStyle,
       paint: _paint,
       textAlign: _textAlign,
@@ -455,7 +471,7 @@ class EngineParagraph implements ui.Paragraph {
 
   @override
   ui.TextPosition getPositionForOffset(ui.Offset offset) {
-    final List<EngineLineMetrics> lines = _measurementResult.lines;
+    final List<EngineLineMetrics>? lines = _measurementResult!.lines;
     if (!_hasLineMetrics) {
       return getPositionForMultiSpanOffset(offset);
     }
@@ -468,12 +484,12 @@ class EngineParagraph implements ui.Paragraph {
       );
     }
 
-    final int lineNumber = offset.dy ~/ _measurementResult.lineHeight;
+    final int lineNumber = offset.dy ~/ _measurementResult!.lineHeight!;
 
     // [offset] is below all the lines.
-    if (lineNumber >= lines.length) {
+    if (lineNumber >= lines!.length) {
       return ui.TextPosition(
-        offset: _plainText.length,
+        offset: _plainText!.length,
         affinity: ui.TextAffinity.upstream,
       );
     }
@@ -562,7 +578,7 @@ class EngineParagraph implements ui.Paragraph {
 
   EngineLineMetrics _getLineForIndex(int index) {
     assert(_hasLineMetrics);
-    final List<EngineLineMetrics> lines = _measurementResult.lines;
+    final List<EngineLineMetrics> lines = _measurementResult!.lines!;
     assert(index >= 0);
 
     for (int i = 0; i < lines.length; i++) {
@@ -586,7 +602,7 @@ class EngineParagraph implements ui.Paragraph {
 
   @override
   List<ui.LineMetrics> computeLineMetrics() {
-    return _measurementResult.lines;
+    return _measurementResult!.lines!;
   }
 }
 
@@ -594,18 +610,18 @@ class EngineParagraph implements ui.Paragraph {
 class EngineParagraphStyle implements ui.ParagraphStyle {
   /// Creates a new instance of [EngineParagraphStyle].
   EngineParagraphStyle({
-    ui.TextAlign textAlign,
-    ui.TextDirection textDirection,
-    int maxLines,
-    String fontFamily,
-    double fontSize,
-    double height,
-    ui.TextHeightBehavior textHeightBehavior,
-    ui.FontWeight fontWeight,
-    ui.FontStyle fontStyle,
-    ui.StrutStyle strutStyle,
-    String ellipsis,
-    ui.Locale locale,
+    ui.TextAlign? textAlign,
+    ui.TextDirection? textDirection,
+    int? maxLines,
+    String? fontFamily,
+    double? fontSize,
+    double? height,
+    ui.TextHeightBehavior? textHeightBehavior,
+    ui.FontWeight? fontWeight,
+    ui.FontStyle? fontStyle,
+    ui.StrutStyle? strutStyle,
+    String? ellipsis,
+    ui.Locale? locale,
   })  : _textAlign = textAlign,
         _textDirection = textDirection,
         _fontWeight = fontWeight,
@@ -616,22 +632,26 @@ class EngineParagraphStyle implements ui.ParagraphStyle {
         _height = height,
         _textHeightBehavior = textHeightBehavior,
         // TODO(b/128317744): add support for strut style.
-        _strutStyle = strutStyle,
+        _strutStyle = strutStyle as EngineStrutStyle?,
         _ellipsis = ellipsis,
         _locale = locale;
 
-  final ui.TextAlign _textAlign;
-  final ui.TextDirection _textDirection;
-  final ui.FontWeight _fontWeight;
-  final ui.FontStyle _fontStyle;
-  final int _maxLines;
-  final String _fontFamily;
-  final double _fontSize;
-  final double _height;
-  final ui.TextHeightBehavior _textHeightBehavior;
-  final EngineStrutStyle _strutStyle;
-  final String _ellipsis;
-  final ui.Locale _locale;
+  final ui.TextAlign? _textAlign;
+  final ui.TextDirection? _textDirection;
+  final ui.FontWeight? _fontWeight;
+  final ui.FontStyle? _fontStyle;
+  final int? _maxLines;
+  final String? _fontFamily;
+  final double? _fontSize;
+  final double? _height;
+  final ui.TextHeightBehavior? _textHeightBehavior;
+  final EngineStrutStyle? _strutStyle;
+  final String? _ellipsis;
+  final ui.Locale? _locale;
+
+  // The effective style attributes should be consistent with paragraph_style.h.
+  ui.TextAlign get _effectiveTextAlign => _textAlign ?? ui.TextAlign.start;
+  ui.TextDirection get _effectiveTextDirection => _textDirection ?? ui.TextDirection.ltr;
 
   String get _effectiveFontFamily {
     if (assertionsEnabled) {
@@ -641,28 +661,29 @@ class EngineParagraphStyle implements ui.ParagraphStyle {
         return 'Ahem';
       }
     }
-    if (_fontFamily == null || _fontFamily.isEmpty) {
+    final String? fontFamily = _fontFamily;
+    if (fontFamily == null || fontFamily.isEmpty) {
       return DomRenderer.defaultFontFamily;
     }
-    return _fontFamily;
+    return fontFamily;
   }
 
-  double get _lineHeight {
+  double? get _lineHeight {
     // TODO(mdebbar): Implement proper support for strut styles.
     // https://github.com/flutter/flutter/issues/32243
     if (_strutStyle == null ||
-        _strutStyle._height == null ||
-        _strutStyle._height == 0) {
+        _strutStyle!._height == null ||
+        _strutStyle!._height == 0) {
       // When there's no strut height, always use paragraph style height.
       return _height;
     }
-    if (_strutStyle._forceStrutHeight == true) {
+    if (_strutStyle!._forceStrutHeight == true) {
       // When strut height is forced, ignore paragraph style height.
-      return _strutStyle._height;
+      return _strutStyle!._height;
     }
     // In this case, strut height acts as a minimum height for all parts of the
     // paragraph. So we take the max of strut height and paragraph style height.
-    return math.max(_strutStyle._height, _height ?? 0.0);
+    return math.max(_strutStyle!._height!, _height ?? 0.0);
   }
 
   @override
@@ -714,8 +735,8 @@ class EngineParagraphStyle implements ui.ParagraphStyle {
           'maxLines: ${_maxLines ?? "unspecified"}, '
           'textHeightBehavior: ${_textHeightBehavior ?? "unspecified"}, '
           'fontFamily: ${_fontFamily ?? "unspecified"}, '
-          'fontSize: ${_fontSize != null ? _fontSize.toStringAsFixed(1) : "unspecified"}, '
-          'height: ${_height != null ? "${_height.toStringAsFixed(1)}x" : "unspecified"}, '
+          'fontSize: ${_fontSize != null ? _fontSize!.toStringAsFixed(1) : "unspecified"}, '
+          'height: ${_height != null ? "${_height!.toStringAsFixed(1)}x" : "unspecified"}, '
           'ellipsis: ${_ellipsis != null ? "\"$_ellipsis\"" : "unspecified"}, '
           'locale: ${_locale ?? "unspecified"}'
           ')';
@@ -728,25 +749,25 @@ class EngineParagraphStyle implements ui.ParagraphStyle {
 /// The web implementation of [ui.TextStyle].
 class EngineTextStyle implements ui.TextStyle {
   EngineTextStyle({
-    ui.Color color,
-    ui.TextDecoration decoration,
-    ui.Color decorationColor,
-    ui.TextDecorationStyle decorationStyle,
-    double decorationThickness,
-    ui.FontWeight fontWeight,
-    ui.FontStyle fontStyle,
-    ui.TextBaseline textBaseline,
-    String fontFamily,
-    List<String> fontFamilyFallback,
-    double fontSize,
-    double letterSpacing,
-    double wordSpacing,
-    double height,
-    ui.Locale locale,
-    ui.Paint background,
-    ui.Paint foreground,
-    List<ui.Shadow> shadows,
-    List<ui.FontFeature> fontFeatures,
+    ui.Color? color,
+    ui.TextDecoration? decoration,
+    ui.Color? decorationColor,
+    ui.TextDecorationStyle? decorationStyle,
+    double? decorationThickness,
+    ui.FontWeight? fontWeight,
+    ui.FontStyle? fontStyle,
+    ui.TextBaseline? textBaseline,
+    String? fontFamily,
+    List<String>? fontFamilyFallback,
+    double? fontSize,
+    double? letterSpacing,
+    double? wordSpacing,
+    double? height,
+    ui.Locale? locale,
+    ui.Paint? background,
+    ui.Paint? foreground,
+    List<ui.Shadow>? shadows,
+    List<ui.FontFeature>? fontFeatures,
   })  : assert(
             color == null || foreground == null,
             'Cannot provide both a color and a foreground\n'
@@ -773,26 +794,26 @@ class EngineTextStyle implements ui.TextStyle {
         _foreground = foreground,
         _shadows = shadows;
 
-  final ui.Color _color;
-  final ui.TextDecoration _decoration;
-  final ui.Color _decorationColor;
-  final ui.TextDecorationStyle _decorationStyle;
-  final double _decorationThickness;
-  final ui.FontWeight _fontWeight;
-  final ui.FontStyle _fontStyle;
-  final ui.TextBaseline _textBaseline;
+  final ui.Color? _color;
+  final ui.TextDecoration? _decoration;
+  final ui.Color? _decorationColor;
+  final ui.TextDecorationStyle? _decorationStyle;
+  final double? _decorationThickness;
+  final ui.FontWeight? _fontWeight;
+  final ui.FontStyle? _fontStyle;
+  final ui.TextBaseline? _textBaseline;
   final bool _isFontFamilyProvided;
   final String _fontFamily;
-  final List<String> _fontFamilyFallback;
-  final List<ui.FontFeature> _fontFeatures;
-  final double _fontSize;
-  final double _letterSpacing;
-  final double _wordSpacing;
-  final double _height;
-  final ui.Locale _locale;
-  final ui.Paint _background;
-  final ui.Paint _foreground;
-  final List<ui.Shadow> _shadows;
+  final List<String>? _fontFamilyFallback;
+  final List<ui.FontFeature>? _fontFeatures;
+  final double? _fontSize;
+  final double? _letterSpacing;
+  final double? _wordSpacing;
+  final double? _height;
+  final ui.Locale? _locale;
+  final ui.Paint? _background;
+  final ui.Paint? _foreground;
+  final List<ui.Shadow>? _shadows;
 
   String get _effectiveFontFamily {
     if (assertionsEnabled) {
@@ -802,7 +823,7 @@ class EngineTextStyle implements ui.TextStyle {
         return 'Ahem';
       }
     }
-    if (_fontFamily == null || _fontFamily.isEmpty) {
+    if (_fontFamily.isEmpty) {
       return DomRenderer.defaultFontFamily;
     }
     return _fontFamily;
@@ -871,12 +892,12 @@ class EngineTextStyle implements ui.TextStyle {
           'fontWeight: ${_fontWeight ?? "unspecified"}, '
           'fontStyle: ${_fontStyle ?? "unspecified"}, '
           'textBaseline: ${_textBaseline ?? "unspecified"}, '
-          'fontFamily: ${_isFontFamilyProvided && _fontFamily != null ? _fontFamily : "unspecified"}, '
-          'fontFamilyFallback: ${_isFontFamilyProvided && _fontFamilyFallback != null && _fontFamilyFallback.isNotEmpty ? _fontFamilyFallback : "unspecified"}, '
-          'fontSize: ${_fontSize != null ? _fontSize.toStringAsFixed(1) : "unspecified"}, '
+          'fontFamily: ${_isFontFamilyProvided && _fontFamily != '' ? _fontFamily : "unspecified"}, '
+          'fontFamilyFallback: ${_isFontFamilyProvided && _fontFamilyFallback != null && _fontFamilyFallback!.isNotEmpty ? _fontFamilyFallback : "unspecified"}, '
+          'fontSize: ${_fontSize != null ? _fontSize!.toStringAsFixed(1) : "unspecified"}, '
           'letterSpacing: ${_letterSpacing != null ? "${_letterSpacing}x" : "unspecified"}, '
           'wordSpacing: ${_wordSpacing != null ? "${_wordSpacing}x" : "unspecified"}, '
-          'height: ${_height != null ? "${_height.toStringAsFixed(1)}x" : "unspecified"}, '
+          'height: ${_height != null ? "${_height!.toStringAsFixed(1)}x" : "unspecified"}, '
           'locale: ${_locale ?? "unspecified"}, '
           'background: ${_background ?? "unspecified"}, '
           'foreground: ${_foreground ?? "unspecified"}, '
@@ -927,14 +948,14 @@ class EngineStrutStyle implements ui.StrutStyle {
   ///   of the [fontFamily] and `(lineHeight + leading) * fontSize`. Otherwise, it
   ///   will be determined by the Ascent + half-leading of the first text.
   EngineStrutStyle({
-    String fontFamily,
-    List<String> fontFamilyFallback,
-    double fontSize,
-    double height,
-    double leading,
-    ui.FontWeight fontWeight,
-    ui.FontStyle fontStyle,
-    bool forceStrutHeight,
+    String? fontFamily,
+    List<String>? fontFamilyFallback,
+    double? fontSize,
+    double? height,
+    double? leading,
+    ui.FontWeight? fontWeight,
+    ui.FontStyle? fontStyle,
+    bool? forceStrutHeight,
   })  : _fontFamily = fontFamily,
         _fontFamilyFallback = fontFamilyFallback,
         _fontSize = fontSize,
@@ -944,14 +965,14 @@ class EngineStrutStyle implements ui.StrutStyle {
         _fontStyle = fontStyle,
         _forceStrutHeight = forceStrutHeight;
 
-  final String _fontFamily;
-  final List<String> _fontFamilyFallback;
-  final double _fontSize;
-  final double _height;
-  final double _leading;
-  final ui.FontWeight _fontWeight;
-  final ui.FontStyle _fontStyle;
-  final bool _forceStrutHeight;
+  final String? _fontFamily;
+  final List<String>? _fontFamilyFallback;
+  final double? _fontSize;
+  final double? _height;
+  final double? _leading;
+  final ui.FontWeight? _fontWeight;
+  final ui.FontStyle? _fontStyle;
+  final bool? _forceStrutHeight;
 
   @override
   bool operator ==(dynamic other) {
@@ -991,7 +1012,7 @@ class EngineParagraphBuilder implements ui.ParagraphBuilder {
   /// Marks a call to the [pop] method in the [_ops] list.
   static final Object _paragraphBuilderPop = Object();
 
-  final html.HtmlElement _paragraphElement = domRenderer.createElement('p');
+  final html.HtmlElement _paragraphElement = domRenderer.createElement('p') as html.HtmlElement;
   final EngineParagraphStyle _paragraphStyle;
   final List<dynamic> _ops = <dynamic>[];
 
@@ -999,14 +1020,14 @@ class EngineParagraphBuilder implements ui.ParagraphBuilder {
   /// [EngineParagraph].
   EngineParagraphBuilder(EngineParagraphStyle style) : _paragraphStyle = style {
     // TODO(b/128317744): Implement support for strut font families.
-    List<String> strutFontFamilies;
+    List<String?> strutFontFamilies;
     if (style._strutStyle != null) {
-      strutFontFamilies = <String>[];
-      if (style._strutStyle._fontFamily != null) {
-        strutFontFamilies.add(style._strutStyle._fontFamily);
+      strutFontFamilies = <String?>[];
+      if (style._strutStyle!._fontFamily != null) {
+        strutFontFamilies.add(style._strutStyle!._fontFamily);
       }
-      if (style._strutStyle._fontFamilyFallback != null) {
-        strutFontFamilies.addAll(style._strutStyle._fontFamilyFallback);
+      if (style._strutStyle!._fontFamilyFallback != null) {
+        strutFontFamilies.addAll(style._strutStyle!._fontFamilyFallback!);
       }
     }
     _applyParagraphStyleToElement(
@@ -1023,7 +1044,7 @@ class EngineParagraphBuilder implements ui.ParagraphBuilder {
 
   @override
   int get placeholderCount => _placeholderCount;
-  int _placeholderCount;
+  late int _placeholderCount;
 
   @override
   List<double> get placeholderScales => _placeholderScales;
@@ -1035,8 +1056,8 @@ class EngineParagraphBuilder implements ui.ParagraphBuilder {
     double height,
     ui.PlaceholderAlignment alignment, {
     double scale = 1.0,
-    double baselineOffset,
-    ui.TextBaseline baseline,
+    double? baselineOffset,
+    ui.TextBaseline? baseline,
   }) {
     // TODO(garyq): Implement stub_ui version of this.
     throw UnimplementedError();
@@ -1090,25 +1111,25 @@ class EngineParagraphBuilder implements ui.ParagraphBuilder {
   /// arbitrarily. However, it may not mix multiple styles in the same
   /// paragraph. Plain text is more efficient to lay out and measure than rich
   /// text.
-  EngineParagraph _tryBuildPlainText() {
-    ui.Color color;
-    ui.TextDecoration decoration;
-    ui.Color decorationColor;
-    ui.TextDecorationStyle decorationStyle;
-    ui.FontWeight fontWeight = _paragraphStyle._fontWeight;
-    ui.FontStyle fontStyle = _paragraphStyle._fontStyle;
-    ui.TextBaseline textBaseline;
-    String fontFamily = _paragraphStyle._fontFamily;
-    double fontSize = _paragraphStyle._fontSize;
-    final ui.TextAlign textAlign = _paragraphStyle._textAlign;
-    final ui.TextDirection textDirection = _paragraphStyle._textDirection;
-    double letterSpacing;
-    double wordSpacing;
-    double height;
-    ui.Locale locale = _paragraphStyle._locale;
-    ui.Paint background;
-    ui.Paint foreground;
-    List<ui.Shadow> shadows;
+  EngineParagraph? _tryBuildPlainText() {
+    ui.Color? color;
+    ui.TextDecoration? decoration;
+    ui.Color? decorationColor;
+    ui.TextDecorationStyle? decorationStyle;
+    ui.FontWeight? fontWeight = _paragraphStyle._fontWeight;
+    ui.FontStyle? fontStyle = _paragraphStyle._fontStyle;
+    ui.TextBaseline? textBaseline;
+    String? fontFamily = _paragraphStyle._fontFamily;
+    double? fontSize = _paragraphStyle._fontSize;
+    final ui.TextAlign textAlign = _paragraphStyle._effectiveTextAlign;
+    final ui.TextDirection textDirection = _paragraphStyle._effectiveTextDirection;
+    double? letterSpacing;
+    double? wordSpacing;
+    double? height;
+    ui.Locale? locale = _paragraphStyle._locale;
+    ui.Paint? background;
+    ui.Paint? foreground;
+    List<ui.Shadow>? shadows;
 
     int i = 0;
 
@@ -1139,9 +1160,7 @@ class EngineParagraphBuilder implements ui.ParagraphBuilder {
       if (style._textBaseline != null) {
         textBaseline = style._textBaseline;
       }
-      if (style._fontFamily != null) {
-        fontFamily = style._fontFamily;
-      }
+      fontFamily = style._fontFamily;
       if (style._fontSize != null) {
         fontSize = style._fontSize;
       }
@@ -1287,7 +1306,7 @@ class EngineParagraphBuilder implements ui.ParagraphBuilder {
     for (int i = 0; i < _ops.length; i++) {
       final dynamic op = _ops[i];
       if (op is EngineTextStyle) {
-        final html.SpanElement span = domRenderer.createElement('span');
+        final html.SpanElement span = domRenderer.createElement('span') as html.SpanElement;
         _applyTextStyleToElement(element: span, style: op, isSpan: true);
         if (op._background != null) {
           _applyTextBackgroundToElement(element: span, style: op);
@@ -1316,15 +1335,15 @@ class EngineParagraphBuilder implements ui.ParagraphBuilder {
       ),
       plainText: null,
       paint: null,
-      textAlign: _paragraphStyle._textAlign,
-      textDirection: _paragraphStyle._textDirection,
+      textAlign: _paragraphStyle._effectiveTextAlign,
+      textDirection: _paragraphStyle._effectiveTextDirection,
       background: null,
     );
   }
 }
 
 /// Converts [fontWeight] to its CSS equivalent value.
-String fontWeightToCss(ui.FontWeight fontWeight) {
+String? fontWeightToCss(ui.FontWeight? fontWeight) {
   if (fontWeight == null) {
     return null;
   }
@@ -1367,12 +1386,12 @@ String fontWeightIndexToCss({int fontWeightIndex = 3}) {
 ///
 /// If [previousStyle] is not null, updates only the mismatching attributes.
 void _applyParagraphStyleToElement({
-  @required html.HtmlElement element,
-  @required EngineParagraphStyle style,
-  EngineParagraphStyle previousStyle,
+  required html.HtmlElement element,
+  required EngineParagraphStyle style,
+  EngineParagraphStyle? previousStyle,
 }) {
-  assert(element != null);
-  assert(style != null);
+  assert(element != null); // ignore: unnecessary_null_comparison
+  assert(style != null); // ignore: unnecessary_null_comparison
   // TODO(yjbanov): What do we do about ParagraphStyle._locale and ellipsis?
   final html.CssStyleDeclaration cssStyle = element.style;
   if (previousStyle == null) {
@@ -1387,7 +1406,7 @@ void _applyParagraphStyleToElement({
       cssStyle.direction = _textDirectionToCss(style._textDirection);
     }
     if (style._fontSize != null) {
-      cssStyle.fontSize = '${style._fontSize.floor()}px';
+      cssStyle.fontSize = '${style._fontSize!.floor()}px';
     }
     if (style._fontWeight != null) {
       cssStyle.fontWeight = fontWeightToCss(style._fontWeight);
@@ -1396,9 +1415,7 @@ void _applyParagraphStyleToElement({
       cssStyle.fontStyle =
           style._fontStyle == ui.FontStyle.normal ? 'normal' : 'italic';
     }
-    if (style._effectiveFontFamily != null) {
-      cssStyle.fontFamily = canonicalizeFontFamily(style._effectiveFontFamily);
-    }
+    cssStyle.fontFamily = canonicalizeFontFamily(style._effectiveFontFamily);
   } else {
     if (style._textAlign != previousStyle._textAlign) {
       cssStyle.textAlign = textAlignToCssValue(
@@ -1412,7 +1429,7 @@ void _applyParagraphStyleToElement({
     }
     if (style._fontSize != previousStyle._fontSize) {
       cssStyle.fontSize =
-          style._fontSize != null ? '${style._fontSize.floor()}px' : null;
+          style._fontSize != null ? '${style._fontSize!.floor()}px' : null;
     }
     if (style._fontWeight != previousStyle._fontWeight) {
       cssStyle.fontWeight = fontWeightToCss(style._fontWeight);
@@ -1435,22 +1452,22 @@ void _applyParagraphStyleToElement({
 /// If [isSpan] is true, the text element is a span within richtext and
 /// should not assign effectiveFontFamily if fontFamily was not specified.
 void _applyTextStyleToElement({
-  @required html.HtmlElement element,
-  @required EngineTextStyle style,
-  EngineTextStyle previousStyle,
+  required html.HtmlElement element,
+  required EngineTextStyle style,
+  EngineTextStyle? previousStyle,
   bool isSpan = false,
 }) {
-  assert(element != null);
-  assert(style != null);
+  assert(element != null); // ignore: unnecessary_null_comparison
+  assert(style != null); // ignore: unnecessary_null_comparison
   bool updateDecoration = false;
   final html.CssStyleDeclaration cssStyle = element.style;
   if (previousStyle == null) {
-    final ui.Color color = style._foreground?.color ?? style._color;
+    final ui.Color? color = style._foreground?.color ?? style._color;
     if (color != null) {
       cssStyle.color = colorToCssString(color);
     }
     if (style._fontSize != null) {
-      cssStyle.fontSize = '${style._fontSize.floor()}px';
+      cssStyle.fontSize = '${style._fontSize!.floor()}px';
     }
     if (style._fontWeight != null) {
       cssStyle.fontWeight = fontWeightToCss(style._fontWeight);
@@ -1462,14 +1479,10 @@ void _applyTextStyleToElement({
     // For test environment use effectiveFontFamily since we need to
     // consistently use Ahem font.
     if (isSpan && !ui.debugEmulateFlutterTesterEnvironment) {
-      if (style._fontFamily != null) {
-        cssStyle.fontFamily = canonicalizeFontFamily(style._fontFamily);
-      }
+      cssStyle.fontFamily = canonicalizeFontFamily(style._fontFamily);
     } else {
-      if (style._effectiveFontFamily != null) {
-        cssStyle.fontFamily =
-            canonicalizeFontFamily(style._effectiveFontFamily);
-      }
+      cssStyle.fontFamily =
+          canonicalizeFontFamily(style._effectiveFontFamily);
     }
     if (style._letterSpacing != null) {
       cssStyle.letterSpacing = '${style._letterSpacing}px';
@@ -1481,18 +1494,18 @@ void _applyTextStyleToElement({
       updateDecoration = true;
     }
     if (style._shadows != null) {
-      cssStyle.textShadow = _shadowListToCss(style._shadows);
+      cssStyle.textShadow = _shadowListToCss(style._shadows!);
     }
   } else {
     if (style._color != previousStyle._color ||
         style._foreground != previousStyle._foreground) {
-      final ui.Color color = style._foreground?.color ?? style._color;
+      final ui.Color? color = style._foreground?.color ?? style._color;
       cssStyle.color = colorToCssString(color);
     }
 
     if (style._fontSize != previousStyle._fontSize) {
       cssStyle.fontSize =
-          style._fontSize != null ? '${style._fontSize.floor()}px' : null;
+          style._fontSize != null ? '${style._fontSize!.floor()}px' : null;
     }
 
     if (style._fontWeight != previousStyle._fontWeight) {
@@ -1519,13 +1532,13 @@ void _applyTextStyleToElement({
       updateDecoration = true;
     }
     if (style._shadows != previousStyle._shadows) {
-      cssStyle.textShadow = _shadowListToCss(style._shadows);
+      cssStyle.textShadow = _shadowListToCss(style._shadows!);
     }
   }
 
   if (updateDecoration) {
     if (style._decoration != null) {
-      final String textDecoration =
+      final String? textDecoration =
           _textDecorationToCssString(style._decoration, style._decorationStyle);
       if (textDecoration != null) {
         if (browserEngine == BrowserEngine.webkit) {
@@ -1534,9 +1547,9 @@ void _applyTextStyleToElement({
         } else {
           cssStyle.textDecoration = textDecoration;
         }
-        final ui.Color decorationColor = style._decorationColor;
+        final ui.Color? decorationColor = style._decorationColor;
         if (decorationColor != null) {
-          cssStyle.textDecorationColor = colorToCssString(decorationColor);
+          cssStyle.textDecorationColor = colorToCssString(decorationColor)!;
         }
       }
     }
@@ -1567,11 +1580,11 @@ String _shadowListToCss(List<ui.Shadow> shadows) {
 /// Applies background color properties in text style to paragraph or span
 /// elements.
 void _applyTextBackgroundToElement({
-  @required html.HtmlElement element,
-  @required EngineTextStyle style,
-  EngineTextStyle previousStyle,
+  required html.HtmlElement element,
+  required EngineTextStyle style,
+  EngineTextStyle? previousStyle,
 }) {
-  final ui.Paint newBackground = style._background;
+  final ui.Paint? newBackground = style._background;
   if (previousStyle == null) {
     if (newBackground != null) {
       domRenderer.setElementStyle(
@@ -1580,14 +1593,14 @@ void _applyTextBackgroundToElement({
   } else {
     if (newBackground != previousStyle._background) {
       domRenderer.setElementStyle(
-          element, 'background-color', colorToCssString(newBackground.color));
+          element, 'background-color', colorToCssString(newBackground!.color));
     }
   }
 }
 
 /// Converts text decoration style to CSS text-decoration-style value.
-String _textDecorationToCssString(
-    ui.TextDecoration decoration, ui.TextDecorationStyle decorationStyle) {
+String? _textDecorationToCssString(
+    ui.TextDecoration? decoration, ui.TextDecorationStyle? decorationStyle) {
   final StringBuffer decorations = StringBuffer();
   if (decoration != null) {
     if (decoration.contains(ui.TextDecoration.underline)) {
@@ -1606,7 +1619,7 @@ String _textDecorationToCssString(
   return decorations.isEmpty ? null : decorations.toString();
 }
 
-String _decorationStyleToCssString(ui.TextDecorationStyle decorationStyle) {
+String? _decorationStyleToCssString(ui.TextDecorationStyle decorationStyle) {
   switch (decorationStyle) {
     case ui.TextDecorationStyle.dashed:
       return 'dashed';
@@ -1630,14 +1643,14 @@ String _decorationStyleToCssString(ui.TextDecorationStyle decorationStyle) {
 /// ```css
 /// direction: rtl;
 /// ```
-String _textDirectionToCss(ui.TextDirection textDirection) {
+String? _textDirectionToCss(ui.TextDirection? textDirection) {
   if (textDirection == null) {
     return null;
   }
   return textDirectionIndexToCss(textDirection.index);
 }
 
-String textDirectionIndexToCss(int textDirectionIndex) {
+String? textDirectionIndexToCss(int textDirectionIndex) {
   switch (textDirectionIndex) {
     case 0:
       return 'rtl';
@@ -1661,7 +1674,7 @@ String textDirectionIndexToCss(int textDirectionIndex) {
 /// ```css
 /// text-align: right;
 /// ```
-String textAlignToCssValue(ui.TextAlign align, ui.TextDirection textDirection) {
+String? textAlignToCssValue(ui.TextAlign? align, ui.TextDirection textDirection) {
   switch (align) {
     case ui.TextAlign.left:
       return 'left';
@@ -1671,24 +1684,23 @@ String textAlignToCssValue(ui.TextAlign align, ui.TextDirection textDirection) {
       return 'center';
     case ui.TextAlign.justify:
       return 'justify';
-    case ui.TextAlign.start:
+    case ui.TextAlign.end:
       switch (textDirection) {
         case ui.TextDirection.ltr:
-          return null; // it's the default
+          return 'end';
         case ui.TextDirection.rtl:
-          return 'right';
+          return 'left';
       }
       break;
-    case ui.TextAlign.end:
+    default: // including ui.TextAlign.start
       switch (textDirection) {
         case ui.TextDirection.ltr:
-          return 'end';
+          return null; // it's the default
         case ui.TextDirection.rtl:
-          return 'left';
+          return 'right';
       }
       break;
   }
-  throw AssertionError('Unsupported TextAlign value $align');
 }
 
 /// Determines if lists [a] and [b] are deep equivalent.
@@ -1696,7 +1708,7 @@ String textAlignToCssValue(ui.TextAlign align, ui.TextDirection textDirection) {
 /// Returns true if the lists are both null, or if they are both non-null, have
 /// the same length, and contain the same elements in the same order. Returns
 /// false otherwise.
-bool _listEquals<T>(List<T> a, List<T> b) {
+bool _listEquals<T>(List<T>? a, List<T>? b) {
   if (a == null) {
     return b == null;
   }
diff --git a/lib/web_ui/lib/src/engine/text/ruler.dart b/lib/web_ui/lib/src/engine/text/ruler.dart
index 38655a528d861..74080b0659e6f 100644
--- a/lib/web_ui/lib/src/engine/text/ruler.dart
+++ b/lib/web_ui/lib/src/engine/text/ruler.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Contains the subset of [ui.ParagraphStyle] properties that affect layout.
@@ -21,28 +21,28 @@ class ParagraphGeometricStyle {
     this.shadows,
   });
 
-  final ui.FontWeight fontWeight;
-  final ui.FontStyle fontStyle;
-  final String fontFamily;
-  final double fontSize;
-  final double lineHeight;
-  final int maxLines;
-  final double letterSpacing;
-  final double wordSpacing;
-  final String decoration;
-  final String ellipsis;
-  final List<ui.Shadow> shadows;
+  final ui.FontWeight? fontWeight;
+  final ui.FontStyle? fontStyle;
+  final String? fontFamily;
+  final double? fontSize;
+  final double? lineHeight;
+  final int? maxLines;
+  final double? letterSpacing;
+  final double? wordSpacing;
+  final String? decoration;
+  final String? ellipsis;
+  final List<ui.Shadow>? shadows;
 
   // Since all fields above are primitives, cache hashcode since ruler lookups
   // use this style as key.
-  int _cachedHashCode;
+  int? _cachedHashCode;
 
   /// Returns the font-family that should be used to style the paragraph. It may
   /// or may not be different from [fontFamily]:
   ///
   /// - Always returns "Ahem" in tests.
   /// - Provides correct defaults when [fontFamily] doesn't have a value.
-  String get effectiveFontFamily {
+  String? get effectiveFontFamily {
     if (assertionsEnabled) {
       // In widget tests we use a predictable-size font "Ahem". This makes
       // widget tests predictable and less flaky.
@@ -50,13 +50,13 @@ class ParagraphGeometricStyle {
         return 'Ahem';
       }
     }
-    if (fontFamily == null || fontFamily.isEmpty) {
+    if (fontFamily == null || fontFamily!.isEmpty) {
       return DomRenderer.defaultFontFamily;
     }
     return fontFamily;
   }
 
-  String _cssFontString;
+  String? _cssFontString;
 
   /// Cached font string that can be used in CSS.
   ///
@@ -83,7 +83,7 @@ class ParagraphGeometricStyle {
     result.write(' ');
 
     if (fontSize != null) {
-      result.write(fontSize.floor());
+      result.write(fontSize!.floor());
       result.write('px');
     } else {
       result.write(DomRenderer.defaultFontSize);
@@ -163,15 +163,15 @@ class TextDimensions {
   TextDimensions(this._element);
 
   final html.HtmlElement _element;
-  html.Rectangle<num> _cachedBoundingClientRect;
+  html.Rectangle<num>? _cachedBoundingClientRect;
 
   /// Attempts to efficiently copy text from [from].
   ///
   /// The primary efficiency gain is from rare occurrence of rich text in
   /// typical apps.
   void updateText(EngineParagraph from, ParagraphGeometricStyle style) {
-    assert(from != null);
-    assert(_element != null);
+    assert(from != null); // ignore: unnecessary_null_comparison
+    assert(_element != null); // ignore: unnecessary_null_comparison
     assert(from._debugHasSameRootStyle(style));
     assert(() {
       final bool wasEmptyOrPlainText = _element.childNodes.isEmpty ||
@@ -188,7 +188,7 @@ class TextDimensions {
     }());
 
     _invalidateBoundsCache();
-    final String plainText = from._plainText;
+    final String? plainText = from._plainText;
     if (plainText != null) {
       // Plain text: just set the string. The paragraph's style is assumed to
       // match the style set on the `element`. Setting text as plain string is
@@ -205,13 +205,13 @@ class TextDimensions {
     } else {
       // Rich text: deeply copy contents. This is the slow case that should be
       // avoided if fast layout performance is desired.
-      final html.Element copy = from._paragraphElement.clone(true);
+      final html.Element copy = from._paragraphElement.clone(true) as html.Element;
       _element.nodes.addAll(copy.nodes);
     }
   }
 
   /// Updated element style width.
-  void updateConstraintWidth(double width, String ellipsis) {
+  void updateConstraintWidth(double width, String? ellipsis) {
     _invalidateBoundsCache();
 
     if (width.isInfinite) {
@@ -246,7 +246,7 @@ class TextDimensions {
   void applyStyle(ParagraphGeometricStyle style) {
     final html.CssStyleDeclaration elementStyle = _element.style;
     elementStyle
-      ..fontSize = style.fontSize != null ? '${style.fontSize.floor()}px' : null
+      ..fontSize = style.fontSize != null ? '${style.fontSize!.floor()}px' : null
       ..fontFamily = canonicalizeFontFamily(style.effectiveFontFamily)
       ..fontWeight =
           style.fontWeight != null ? fontWeightToCss(style.fontWeight) : null
@@ -257,7 +257,7 @@ class TextDimensions {
           style.letterSpacing != null ? '${style.letterSpacing}px' : null
       ..wordSpacing =
           style.wordSpacing != null ? '${style.wordSpacing}px' : null;
-    final String decoration = style.decoration;
+    final String? decoration = style.decoration;
     if (browserEngine == BrowserEngine.webkit) {
       domRenderer.setElementStyle(
           _element, '-webkit-text-decoration', decoration);
@@ -265,7 +265,7 @@ class TextDimensions {
       elementStyle.textDecoration = decoration;
     }
     if (style.lineHeight != null) {
-      elementStyle.lineHeight = style.lineHeight.toString();
+      elementStyle.lineHeight = style.lineHeight!.toString();
     }
     _invalidateBoundsCache();
   }
@@ -281,11 +281,11 @@ class TextDimensions {
       _cachedBoundingClientRect ??= _element.getBoundingClientRect();
 
   /// The width of the paragraph being measured.
-  double get width => _readAndCacheMetrics().width;
+  double get width => _readAndCacheMetrics().width as double;
 
   /// The height of the paragraph being measured.
   double get height {
-    double cachedHeight = _readAndCacheMetrics().height;
+    double cachedHeight = _readAndCacheMetrics().height as double;
     if (browserEngine == BrowserEngine.firefox &&
       // In the flutter tester environment, we use a predictable-size for font
       // measurement tests.
@@ -356,7 +356,7 @@ class ParagraphRuler {
   final html.HtmlElement _probe = html.DivElement();
 
   /// Cached value of alphabetic base line.
-  double _cachedAlphabeticBaseline;
+  double? _cachedAlphabeticBaseline;
 
   ParagraphRuler(this.style, this.rulerManager) {
     _configureSingleLineHostElements();
@@ -370,7 +370,7 @@ class ParagraphRuler {
 
   /// The alphabetic baseline of the paragraph being measured.
   double get alphabeticBaseline =>
-      _cachedAlphabeticBaseline ??= _probe.getBoundingClientRect().bottom;
+      (_cachedAlphabeticBaseline ??= _probe.getBoundingClientRect().bottom as double?)!;
 
   // Elements used to measure single-line metrics.
   final html.DivElement _singleLineHost = html.DivElement();
@@ -388,16 +388,16 @@ class ParagraphRuler {
       TextDimensions(html.ParagraphElement());
 
   // Elements used to measure the line-height metric.
-  html.DivElement _lineHeightHost;
-  TextDimensions _lineHeightDimensions;
-  TextDimensions get lineHeightDimensions {
+  html.DivElement? _lineHeightHost;
+  TextDimensions? _lineHeightDimensions;
+  TextDimensions? get lineHeightDimensions {
     // Lazily create the elements for line-height measurement since they are not
     // always needed.
     if (_lineHeightDimensions == null) {
       _lineHeightHost = html.DivElement();
       _lineHeightDimensions = TextDimensions(html.ParagraphElement());
       _configureLineHeightHostElements();
-      _lineHeightHost.append(_probe);
+      _lineHeightHost!.append(_probe);
     }
     return _lineHeightDimensions;
   }
@@ -523,7 +523,7 @@ class ParagraphRuler {
   }
 
   void _configureLineHeightHostElements() {
-    _lineHeightHost.style
+    _lineHeightHost!.style
       ..visibility = 'hidden'
       ..position = 'absolute'
       ..top = '0'
@@ -536,29 +536,29 @@ class ParagraphRuler {
       ..padding = '0';
 
     if (assertionsEnabled) {
-      _lineHeightHost.setAttribute('data-ruler', 'line-height');
+      _lineHeightHost!.setAttribute('data-ruler', 'line-height');
     }
 
-    lineHeightDimensions.applyStyle(style);
+    lineHeightDimensions!.applyStyle(style);
 
     // Force single-line (even if wider than screen) and preserve whitespaces.
-    lineHeightDimensions._element.style.whiteSpace = 'pre';
+    lineHeightDimensions!._element.style.whiteSpace = 'pre';
 
     // To measure line-height, all we need is a whitespace.
-    lineHeightDimensions.updateTextToSpace();
+    lineHeightDimensions!.updateTextToSpace();
 
-    lineHeightDimensions.appendToHost(_lineHeightHost);
-    rulerManager.addHostElement(_lineHeightHost);
+    lineHeightDimensions!.appendToHost(_lineHeightHost!);
+    rulerManager.addHostElement(_lineHeightHost!);
   }
 
   /// The paragraph being measured.
-  EngineParagraph _paragraph;
+  EngineParagraph? _paragraph;
 
   /// Prepares this ruler for measuring the given [paragraph].
   ///
   /// This method must be called before calling any of the `measure*` methods.
   void willMeasure(EngineParagraph paragraph) {
-    assert(paragraph != null);
+    assert(paragraph != null); // ignore: unnecessary_null_comparison
     assert(() {
       if (_paragraph != null) {
         throw Exception(
@@ -593,10 +593,10 @@ class ParagraphRuler {
     // which doesn't work. So we need to replace it with a whitespace. The
     // correct fix would be to do line height and baseline measurements and
     // cache them separately.
-    if (_paragraph._plainText == '') {
+    if (_paragraph!._plainText == '') {
       singleLineDimensions.updateTextToSpace();
     } else {
-      singleLineDimensions.updateText(_paragraph, style);
+      singleLineDimensions.updateText(_paragraph!, style);
     }
   }
 
@@ -608,7 +608,7 @@ class ParagraphRuler {
     assert(!_debugIsDisposed);
     assert(_paragraph != null);
 
-    minIntrinsicDimensions.updateText(_paragraph, style);
+    minIntrinsicDimensions.updateText(_paragraph!, style);
   }
 
   /// Lays out the paragraph giving it a width constraint.
@@ -618,7 +618,7 @@ class ParagraphRuler {
     assert(!_debugIsDisposed);
     assert(_paragraph != null);
 
-    constrainedDimensions.updateText(_paragraph, style);
+    constrainedDimensions.updateText(_paragraph!, style);
 
     // The extra 0.5 is because sometimes the browser needs slightly more space
     // than the size it reports back. When that happens the text may be wrap
@@ -645,7 +645,7 @@ class ParagraphRuler {
       final html.Node node = textNodes[i];
       // Check if offset is within client rect bounds of text node's
       // parent element.
-      final html.Element parent = node.parentNode;
+      final html.Element parent = node.parentNode as html.Element;
       final html.Rectangle<num> bounds = parent.getBoundingClientRect();
       final double dx = offset.dx;
       final double dy = offset.dy;
@@ -685,7 +685,7 @@ class ParagraphRuler {
         break;
       }
       if (node.nodeType == html.Node.TEXT_NODE) {
-        position += node.text.length;
+        position += node.text!.length;
       }
     }
     return position;
@@ -709,7 +709,7 @@ class ParagraphRuler {
     //
     // We do not do this for plain text, because replacing plain text is more
     // expensive than paying the cost of the DOM mutation to clean it.
-    if (_paragraph._plainText == null) {
+    if (_paragraph!._plainText == null) {
       domRenderer
         ..clearDom(singleLineDimensions._element)
         ..clearDom(minIntrinsicDimensions._element)
@@ -725,10 +725,10 @@ class ParagraphRuler {
   List<ui.TextBox> measureBoxesForRange(
     String plainText,
     ui.ParagraphConstraints constraints, {
-    int start,
-    int end,
-    double alignOffset,
-    ui.TextDirection textDirection,
+    required int start,
+    required int end,
+    required double alignOffset,
+    required ui.TextDirection textDirection,
   }) {
     assert(!_debugIsDisposed);
     assert(start >= 0 && start <= plainText.length);
@@ -755,9 +755,9 @@ class ParagraphRuler {
 
     final double maxLinesLimit = style.maxLines == null
         ? double.infinity
-        : style.maxLines * lineHeightDimensions.height;
+        : style.maxLines! * lineHeightDimensions!.height;
 
-    html.Rectangle<num> previousRect;
+    html.Rectangle<num>? previousRect;
     for (html.Rectangle<num> rect in clientRects) {
       // If [rect] is an empty box on the same line as the previous box, don't
       // include it in the result.
@@ -770,10 +770,10 @@ class ParagraphRuler {
       }
 
       boxes.add(ui.TextBox.fromLTRBD(
-        rect.left + alignOffset,
-        rect.top,
-        rect.right + alignOffset,
-        rect.bottom,
+        rect.left + alignOffset as double,
+        rect.top as double,
+        rect.right + alignOffset as double,
+        rect.bottom as double,
         textDirection,
       ));
       previousRect = rect;
@@ -807,10 +807,10 @@ class ParagraphRuler {
   }
 
   // Bounded cache for text measurement for a particular width constraint.
-  Map<String, List<MeasurementResult>> _measurementCache =
-      <String, List<MeasurementResult>>{};
+  Map<String?, List<MeasurementResult?>> _measurementCache =
+      <String?, List<MeasurementResult?>>{};
   // Mru list for cache.
-  final List<String> _mruList = <String>[];
+  final List<String?> _mruList = <String?>[];
   static const int _cacheLimit = 2400;
   // Number of items to evict when cache limit is reached.
   static const int _cacheBlockFactor = 100;
@@ -819,10 +819,10 @@ class ParagraphRuler {
   // is changing.
   static const int _constraintCacheSize = 8;
 
-  void cacheMeasurement(EngineParagraph paragraph, MeasurementResult item) {
-    final String plainText = paragraph._plainText;
-    final List<MeasurementResult> constraintCache =
-        _measurementCache[plainText] ??= <MeasurementResult>[];
+  void cacheMeasurement(EngineParagraph paragraph, MeasurementResult? item) {
+    final String? plainText = paragraph._plainText;
+    final List<MeasurementResult?> constraintCache =
+        _measurementCache[plainText] ??= <MeasurementResult?>[];
     constraintCache.add(item);
     if (constraintCache.length > _constraintCacheSize) {
       constraintCache.removeAt(0);
@@ -837,21 +837,21 @@ class ParagraphRuler {
     }
   }
 
-  MeasurementResult cacheLookup(
+  MeasurementResult? cacheLookup(
       EngineParagraph paragraph, ui.ParagraphConstraints constraints) {
-    final String plainText = paragraph._plainText;
+    final String? plainText = paragraph._plainText;
     if (plainText == null) {
       // Multi span paragraph, do not use cache item.
       return null;
     }
-    final List<MeasurementResult> constraintCache =
+    final List<MeasurementResult?>? constraintCache =
         _measurementCache[plainText];
     if (constraintCache == null) {
       return null;
     }
     final int len = constraintCache.length;
     for (int i = 0; i < len; i++) {
-      final MeasurementResult item = constraintCache[i];
+      final MeasurementResult item = constraintCache[i]!;
       if (item.constraintWidth == constraints.width &&
           item.textAlign == paragraph._textAlign &&
           item.textDirection == paragraph._textDirection) {
@@ -888,7 +888,7 @@ class MeasurementResult {
   ///
   /// In some cases, measuring [lineHeight] is unnecessary, so it's nullable. If
   /// present, it should be equal to [height] when [isSingleLine] is true.
-  final double lineHeight;
+  final double? lineHeight;
 
   /// {@macro dart.ui.paragraph.minIntrinsicWidth}
   final double minIntrinsicWidth;
@@ -904,7 +904,7 @@ class MeasurementResult {
 
   /// The full list of [EngineLineMetrics] that describe in detail the various metrics
   /// of each laid out line.
-  final List<EngineLineMetrics> lines;
+  final List<EngineLineMetrics>? lines;
 
   /// The text align value of the paragraph.
   final ui.TextAlign textAlign;
@@ -914,27 +914,27 @@ class MeasurementResult {
 
   MeasurementResult(
     this.constraintWidth, {
-    @required this.isSingleLine,
-    @required this.width,
-    @required this.height,
-    @required this.naturalHeight,
-    @required this.lineHeight,
-    @required this.minIntrinsicWidth,
-    @required this.maxIntrinsicWidth,
-    @required this.alphabeticBaseline,
-    @required this.ideographicBaseline,
-    @required this.lines,
-    @required ui.TextAlign textAlign,
-    @required ui.TextDirection textDirection,
-  })  : assert(constraintWidth != null),
-        assert(isSingleLine != null),
-        assert(width != null),
-        assert(height != null),
-        assert(naturalHeight != null),
-        assert(minIntrinsicWidth != null),
-        assert(maxIntrinsicWidth != null),
-        assert(alphabeticBaseline != null),
-        assert(ideographicBaseline != null),
+    required this.isSingleLine,
+    required this.width,
+    required this.height,
+    required this.naturalHeight,
+    required this.lineHeight,
+    required this.minIntrinsicWidth,
+    required this.maxIntrinsicWidth,
+    required this.alphabeticBaseline,
+    required this.ideographicBaseline,
+    required this.lines,
+    required ui.TextAlign? textAlign,
+    required ui.TextDirection? textDirection,
+  })  : assert(constraintWidth != null), // ignore: unnecessary_null_comparison
+        assert(isSingleLine != null), // ignore: unnecessary_null_comparison
+        assert(width != null), // ignore: unnecessary_null_comparison
+        assert(height != null), // ignore: unnecessary_null_comparison
+        assert(naturalHeight != null), // ignore: unnecessary_null_comparison
+        assert(minIntrinsicWidth != null), // ignore: unnecessary_null_comparison
+        assert(maxIntrinsicWidth != null), // ignore: unnecessary_null_comparison
+        assert(alphabeticBaseline != null), // ignore: unnecessary_null_comparison
+        assert(ideographicBaseline != null), // ignore: unnecessary_null_comparison
         this.textAlign = textAlign ?? ui.TextAlign.start,
         this.textDirection = textDirection ?? ui.TextDirection.ltr;
 }
diff --git a/lib/web_ui/lib/src/engine/text/unicode_range.dart b/lib/web_ui/lib/src/engine/text/unicode_range.dart
index 10263195ac80f..ada988e60c743 100644
--- a/lib/web_ui/lib/src/engine/text/unicode_range.dart
+++ b/lib/web_ui/lib/src/engine/text/unicode_range.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 const int _kChar_0 = 48;
@@ -88,8 +88,8 @@ class UnicodePropertyLookup<P> {
   /// located at that [index].
   ///
   /// If the [index] is out of range, null will be returned.
-  P find(String text, int index) {
-    if (index < 0 || index >= text.length) {
+  P? find(String? text, int index) {
+    if (index < 0 || index >= text!.length) {
       return null;
     }
     return findForChar(text.codeUnitAt(index));
@@ -99,7 +99,7 @@ class UnicodePropertyLookup<P> {
   ///
   /// If a property can't be found for the given character, null will be
   /// returned.
-  P findForChar(int char) {
+  P? findForChar(int char) {
     final int rangeIndex = _binarySearch(char);
     return rangeIndex == -1 ? null : ranges[rangeIndex].property;
   }
diff --git a/lib/web_ui/lib/src/engine/text/word_break_properties.dart b/lib/web_ui/lib/src/engine/text/word_break_properties.dart
index ab8b9746fc6ee..3b634e59bb422 100644
--- a/lib/web_ui/lib/src/engine/text/word_break_properties.dart
+++ b/lib/web_ui/lib/src/engine/text/word_break_properties.dart
@@ -12,7 +12,7 @@
 // # Unicode and the Unicode Logo are registered trademarks of Unicode, Inc. in the U.S. and other countries.
 // # For terms of use, see http://www.unicode.org/terms_of_use.html
 
-// @dart = 2.6
+
 part of engine;
 
 /// For an explanation of these enum values, see:
diff --git a/lib/web_ui/lib/src/engine/text/word_breaker.dart b/lib/web_ui/lib/src/engine/text/word_breaker.dart
index 6df70db3f1a50..037cf058aab25 100644
--- a/lib/web_ui/lib/src/engine/text/word_breaker.dart
+++ b/lib/web_ui/lib/src/engine/text/word_breaker.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 enum _FindBreakDirection {
@@ -16,28 +16,28 @@ enum _FindBreakDirection {
 /// [WordBreaker] exposes static methods to identify word boundaries.
 abstract class WordBreaker {
   /// It starts from [index] and tries to find the next word boundary in [text].
-  static int nextBreakIndex(String text, int index) =>
+  static int nextBreakIndex(String? text, int index) =>
       _findBreakIndex(_FindBreakDirection.forward, text, index);
 
   /// It starts from [index] and tries to find the previous word boundary in
   /// [text].
-  static int prevBreakIndex(String text, int index) =>
+  static int prevBreakIndex(String? text, int index) =>
       _findBreakIndex(_FindBreakDirection.backward, text, index);
 
   static int _findBreakIndex(
     _FindBreakDirection direction,
-    String text,
+    String? text,
     int index,
   ) {
     int step, min, max;
     if (direction == _FindBreakDirection.forward) {
       step = 1;
       min = 0;
-      max = text.length - 1;
+      max = text!.length - 1;
     } else {
       step = -1;
       min = 1;
-      max = text.length;
+      max = text!.length;
     }
 
     int i = index;
@@ -52,11 +52,11 @@ abstract class WordBreaker {
 
   /// Find out if there's a word break between [index - 1] and [index].
   /// http://unicode.org/reports/tr29/#Word_Boundary_Rules
-  static bool _isBreak(String text, int index) {
+  static bool _isBreak(String? text, int index) {
     // Break at the start and end of text.
     // WB1: sot ÷ Any
     // WB2: Any ÷ eot
-    if (index <= 0 || index >= text.length) {
+    if (index <= 0 || index >= text!.length) {
       return true;
     }
 
@@ -65,8 +65,8 @@ abstract class WordBreaker {
       return false;
     }
 
-    final WordCharProperty immediateRight = wordLookup.find(text, index);
-    WordCharProperty immediateLeft = wordLookup.find(text, index - 1);
+    final WordCharProperty? immediateRight = wordLookup.find(text, index);
+    WordCharProperty? immediateLeft = wordLookup.find(text, index - 1);
 
     // Do not break within CRLF.
     // WB3: CR × LF
@@ -145,7 +145,7 @@ abstract class WordBreaker {
 
     // Skip all Format, Extend and ZWJ to the right.
     int r = 0;
-    WordCharProperty nextRight;
+    WordCharProperty? nextRight;
     do {
       r++;
       nextRight = wordLookup.find(text, index + r);
@@ -157,7 +157,7 @@ abstract class WordBreaker {
     ));
 
     // Skip all Format, Extend and ZWJ to the left.
-    WordCharProperty nextLeft;
+    WordCharProperty? nextLeft;
     do {
       l++;
       nextLeft = wordLookup.find(text, index - l - 1);
@@ -306,12 +306,12 @@ abstract class WordBreaker {
   }
 
   static bool _oneOf(
-    WordCharProperty value,
+    WordCharProperty? value,
     WordCharProperty choice1,
     WordCharProperty choice2, [
-    WordCharProperty choice3,
-    WordCharProperty choice4,
-    WordCharProperty choice5,
+    WordCharProperty? choice3,
+    WordCharProperty? choice4,
+    WordCharProperty? choice5,
   ]) {
     if (value == choice1) {
       return true;
@@ -331,7 +331,7 @@ abstract class WordBreaker {
     return false;
   }
 
-  static bool _isAHLetter(WordCharProperty property) {
+  static bool _isAHLetter(WordCharProperty? property) {
     return _oneOf(property, WordCharProperty.ALetter, WordCharProperty.HebrewLetter);
   }
 }
diff --git a/lib/web_ui/lib/src/engine/text_editing/autofill_hint.dart b/lib/web_ui/lib/src/engine/text_editing/autofill_hint.dart
index 2b28aee67773e..5e95b45f75f72 100644
--- a/lib/web_ui/lib/src/engine/text_editing/autofill_hint.dart
+++ b/lib/web_ui/lib/src/engine/text_editing/autofill_hint.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Maps AutofillHints from the framework to the autofill hints that is used for
diff --git a/lib/web_ui/lib/src/engine/text_editing/input_type.dart b/lib/web_ui/lib/src/engine/text_editing/input_type.dart
index a3b46ee6db3e3..f4a84b3c65551 100644
--- a/lib/web_ui/lib/src/engine/text_editing/input_type.dart
+++ b/lib/web_ui/lib/src/engine/text_editing/input_type.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Various types of inputs used in text fields.
@@ -57,7 +57,7 @@ abstract class EngineInputType {
   ///
   /// For various `inputmode` values supported by browsers, see:
   /// <https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/inputmode>.
-  String get inputmodeAttribute;
+  String? get inputmodeAttribute;
 
   /// Whether this input type allows the "Enter" key to submit the input action.
   bool get submitActionOnEnter => true;
@@ -75,7 +75,7 @@ abstract class EngineInputType {
     // keyboard shows up.
     if (operatingSystem == OperatingSystem.iOs ||
         operatingSystem == OperatingSystem.android) {
-      domElement.setAttribute('inputmode', inputmodeAttribute);
+      domElement.setAttribute('inputmode', inputmodeAttribute!);
     }
   }
 }
@@ -125,7 +125,7 @@ class MultilineInputType extends EngineInputType {
   const MultilineInputType();
 
   @override
-  final String inputmodeAttribute = null;
+  final String? inputmodeAttribute = null;
 
   @override
   bool get submitActionOnEnter => false;
diff --git a/lib/web_ui/lib/src/engine/text_editing/text_editing.dart b/lib/web_ui/lib/src/engine/text_editing/text_editing.dart
index 607bffc766fe1..abc9f4c50231c 100644
--- a/lib/web_ui/lib/src/engine/text_editing/text_editing.dart
+++ b/lib/web_ui/lib/src/engine/text_editing/text_editing.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Make the content editable span visible to facilitate debugging.
@@ -84,15 +84,15 @@ void _hideAutofillElements(html.HtmlElement domElement) {
 class EngineAutofillForm {
   EngineAutofillForm({this.formElement, this.elements, this.items});
 
-  final html.FormElement formElement;
+  final html.FormElement? formElement;
 
-  final Map<String, html.HtmlElement> elements;
+  final Map<String, html.HtmlElement>? elements;
 
-  final Map<String, AutofillInfo> items;
+  final Map<String, AutofillInfo>? items;
 
-  factory EngineAutofillForm.fromFrameworkMessage(
-    Map<String, dynamic> focusedElementAutofill,
-    List<dynamic> fields,
+  static EngineAutofillForm? fromFrameworkMessage(
+    Map<String, dynamic>? focusedElementAutofill,
+    List<dynamic>? fields,
   ) {
     // Autofill value can be null if focused text element does not have an
     // autofill hint set.
@@ -103,9 +103,6 @@ class EngineAutofillForm {
     // If there is only one text field in the autofill model, `fields` will be
     // null. `focusedElementAutofill` contains the information about the one
     // text field.
-    final bool singleElement = (fields == null);
-    final AutofillInfo focusedElement =
-        AutofillInfo.fromFrameworkMessage(focusedElementAutofill);
     final Map<String, html.HtmlElement> elements = <String, html.HtmlElement>{};
     final Map<String, AutofillInfo> items = <String, AutofillInfo>{};
     final html.FormElement formElement = html.FormElement();
@@ -115,13 +112,15 @@ class EngineAutofillForm {
 
     _hideAutofillElements(formElement);
 
-    if (!singleElement) {
-      for (Map<String, dynamic> field in fields) {
+    if (fields != null) {
+      for (Map<String, dynamic> field in fields.cast<Map<String, dynamic>>()) {
         final Map<String, dynamic> autofillInfo = field['autofill'];
         final AutofillInfo autofill =
             AutofillInfo.fromFrameworkMessage(autofillInfo);
 
         // The focused text editing element will not be created here.
+        final AutofillInfo focusedElement =
+            AutofillInfo.fromFrameworkMessage(focusedElementAutofill);
         if (autofill.uniqueIdentifier != focusedElement.uniqueIdentifier) {
           EngineInputType engineInputType =
               EngineInputType.fromName(field['inputType']['name']);
@@ -146,12 +145,12 @@ class EngineAutofillForm {
   }
 
   void placeForm(html.HtmlElement mainTextEditingElement) {
-    formElement.append(mainTextEditingElement);
-    domRenderer.glassPaneElement.append(formElement);
+    formElement!.append(mainTextEditingElement);
+    domRenderer.glassPaneElement!.append(formElement!);
   }
 
   void removeForm() {
-    formElement.remove();
+    formElement!.remove();
   }
 
   /// Listens to `onInput` event on the form fields.
@@ -165,11 +164,11 @@ class EngineAutofillForm {
   /// [TextEditingStrategy.addEventHandlers] method call and all
   /// listeners are removed during [TextEditingStrategy.disable] method call.
   List<StreamSubscription<html.Event>> addInputEventListeners() {
-    Iterable<String> keys = elements.keys;
+    Iterable<String> keys = elements!.keys;
     List<StreamSubscription<html.Event>> subscriptions =
         <StreamSubscription<html.Event>>[];
     keys.forEach((String key) {
-      final html.Element element = elements[key];
+      final html.Element element = elements![key]!;
       subscriptions.add(element.onInput.listen((html.Event e) {
         _handleChange(element, key);
       }));
@@ -177,14 +176,14 @@ class EngineAutofillForm {
     return subscriptions;
   }
 
-  void _handleChange(html.Element domElement, String tag) {
-    EditingState newEditingState = EditingState.fromDomElement(domElement);
+  void _handleChange(html.Element domElement, String? tag) {
+    EditingState newEditingState = EditingState.fromDomElement(domElement as html.HtmlElement?);
 
     _sendAutofillEditingState(tag, newEditingState);
   }
 
   /// Sends the 'TextInputClient.updateEditingStateWithTag' message to the framework.
-  void _sendAutofillEditingState(String tag, EditingState editingState) {
+  void _sendAutofillEditingState(String? tag, EditingState editingState) {
     if (window._onPlatformMessage != null) {
       window.invokeOnPlatformMessage(
         'flutter/textinput',
@@ -193,7 +192,7 @@ class EngineAutofillForm {
             'TextInputClient.updateEditingStateWithTag',
             <dynamic>[
               0,
-              <String, dynamic>{tag: editingState.toFlutter()}
+              <String?, dynamic>{tag: editingState.toFlutter()}
             ],
           ),
         ),
@@ -208,7 +207,7 @@ class EngineAutofillForm {
 /// These values are to be used when a text field have autofill enabled.
 @visibleForTesting
 class AutofillInfo {
-  AutofillInfo({this.editingState, this.uniqueIdentifier, this.hint});
+  AutofillInfo({required this.editingState, required this.uniqueIdentifier, required this.hint});
 
   /// The current text and selection state of a text field.
   final EditingState editingState;
@@ -226,12 +225,8 @@ class AutofillInfo {
   final String hint;
 
   factory AutofillInfo.fromFrameworkMessage(Map<String, dynamic> autofill) {
-    // Autofill value can be null if no TextFields is set with autofill hint.
-    if (autofill == null) {
-      return null;
-    }
-
-    final String uniqueIdentifier = autofill['uniqueIdentifier'];
+    assert(autofill != null); // ignore: unnecessary_null_comparison
+    final String uniqueIdentifier = autofill['uniqueIdentifier']!;
     final List<dynamic> hintsList = autofill['hints'];
     final EditingState editingState =
         EditingState.fromFrameworkMessage(autofill['editingValue']);
@@ -294,7 +289,7 @@ class EditingState {
       Map<String, dynamic> flutterEditingState) {
     final int selectionBase = flutterEditingState['selectionBase'];
     final int selectionExtent = flutterEditingState['selectionExtent'];
-    final String text = flutterEditingState['text'];
+    final String? text = flutterEditingState['text'];
 
     return EditingState(
         text: text,
@@ -307,7 +302,7 @@ class EditingState {
   ///
   /// [domElement] can be a [InputElement] or a [TextAreaElement] depending on
   /// the [InputType] of the text field.
-  factory EditingState.fromDomElement(html.HtmlElement domElement) {
+  factory EditingState.fromDomElement(html.HtmlElement? domElement) {
     if (domElement is html.InputElement) {
       html.InputElement element = domElement;
       return EditingState(
@@ -335,16 +330,16 @@ class EditingState {
       };
 
   /// The current text being edited.
-  final String text;
+  final String? text;
 
   /// The offset at which the text selection originates.
-  final int baseOffset;
+  final int? baseOffset;
 
   /// The offset at which the text selection terminates.
-  final int extentOffset;
+  final int? extentOffset;
 
   /// Whether the current editing state is valid or not.
-  bool get isValid => baseOffset >= 0 && extentOffset >= 0;
+  bool get isValid => baseOffset! >= 0 && extentOffset! >= 0;
 
   @override
   int get hashCode => ui.hashValues(text, baseOffset, extentOffset);
@@ -374,15 +369,15 @@ class EditingState {
   ///
   /// [domElement] can be a [InputElement] or a [TextAreaElement] depending on
   /// the [InputType] of the text field.
-  void applyToDomElement(html.HtmlElement domElement) {
+  void applyToDomElement(html.HtmlElement? domElement) {
     if (domElement is html.InputElement) {
       html.InputElement element = domElement;
       element.value = text;
-      element.setSelectionRange(baseOffset, extentOffset);
+      element.setSelectionRange(baseOffset!, extentOffset!);
     } else if (domElement is html.TextAreaElement) {
       html.TextAreaElement element = domElement;
       element.value = text;
-      element.setSelectionRange(baseOffset, extentOffset);
+      element.setSelectionRange(baseOffset!, extentOffset!);
     } else {
       throw UnsupportedError('Unsupported DOM element type');
     }
@@ -397,13 +392,14 @@ class EditingState {
 /// This corresponds to Flutter's [TextInputConfiguration].
 class InputConfiguration {
   InputConfiguration({
-    @required this.inputType,
-    @required this.inputAction,
-    @required this.obscureText,
-    @required this.autocorrect,
+    required this.inputType,
+    required this.inputAction,
+    required this.obscureText,
+    required this.autocorrect,
     this.autofill,
     this.autofillGroup,
   });
+
   InputConfiguration.fromFrameworkMessage(
       Map<String, dynamic> flutterInputConfiguration)
       : inputType = EngineInputType.fromName(
@@ -411,8 +407,9 @@ class InputConfiguration {
         inputAction = flutterInputConfiguration['inputAction'],
         obscureText = flutterInputConfiguration['obscureText'],
         autocorrect = flutterInputConfiguration['autocorrect'],
-        autofill = AutofillInfo.fromFrameworkMessage(
-            flutterInputConfiguration['autofill']),
+        autofill = flutterInputConfiguration.containsKey('autofill')
+          ? AutofillInfo.fromFrameworkMessage(flutterInputConfiguration['autofill'])
+          : null,
         autofillGroup = EngineAutofillForm.fromFrameworkMessage(
             flutterInputConfiguration['autofill'],
             flutterInputConfiguration['fields']);
@@ -421,10 +418,10 @@ class InputConfiguration {
   final EngineInputType inputType;
 
   /// The default action for the input field.
-  final String inputAction;
+  final String? inputAction;
 
   /// Whether to hide the text being edited.
-  final bool obscureText;
+  final bool? obscureText;
 
   /// Whether to enable autocorrection.
   ///
@@ -433,15 +430,15 @@ class InputConfiguration {
   ///
   /// For future manual tests, note that autocorrect is an attribute only
   /// supported by Safari.
-  final bool autocorrect;
+  final bool? autocorrect;
 
-  final AutofillInfo autofill;
+  final AutofillInfo? autofill;
 
-  final EngineAutofillForm autofillGroup;
+  final EngineAutofillForm? autofillGroup;
 }
 
-typedef _OnChangeCallback = void Function(EditingState editingState);
-typedef _OnActionCallback = void Function(String inputAction);
+typedef _OnChangeCallback = void Function(EditingState? editingState);
+typedef _OnActionCallback = void Function(String? inputAction);
 
 /// Provides HTML DOM functionality for editable text.
 ///
@@ -450,8 +447,8 @@ typedef _OnActionCallback = void Function(String inputAction);
 abstract class TextEditingStrategy {
   void initializeTextEditing(
     InputConfiguration inputConfig, {
-    @required _OnChangeCallback onChange,
-    @required _OnActionCallback onAction,
+    required _OnChangeCallback onChange,
+    required _OnActionCallback onAction,
   });
 
   /// Sets the initial placement of the DOM element on the UI.
@@ -503,7 +500,7 @@ class GloballyPositionedTextEditingStrategy extends DefaultTextEditingStrategy {
   void placeElement() {
     super.placeElement();
     if (hasAutofillGroup) {
-       _geometry?.applyToDomElement(focusedFormElement);
+       _geometry?.applyToDomElement(focusedFormElement!);
        placeForm();
        // On Chrome, when a form is focused, it opens an autofill menu
        // immeddiately.
@@ -514,7 +511,7 @@ class GloballyPositionedTextEditingStrategy extends DefaultTextEditingStrategy {
        //  `setEditableSizeAndTransform` method is called and focus on the form
        // only after placing it to the correct position. Hence autofill menu
        // does not appear on top-left of the page.
-       focusedFormElement.focus();
+       focusedFormElement!.focus();
     } else {
       _geometry?.applyToDomElement(domElement);
     }
@@ -550,45 +547,50 @@ abstract class DefaultTextEditingStrategy implements TextEditingStrategy {
   @visibleForTesting
   bool isEnabled = false;
 
-  html.HtmlElement domElement;
-  InputConfiguration _inputConfiguration;
-  EditingState _lastEditingState;
+  html.HtmlElement get domElement => _domElement!;
+  set domElement(html.HtmlElement element) {
+    _domElement = element;
+  }
+  html.HtmlElement? _domElement;
+
+  late InputConfiguration _inputConfiguration;
+  EditingState? _lastEditingState;
 
   /// Styles associated with the editable text.
-  EditableTextStyle _style;
+  EditableTextStyle? _style;
 
   /// Size and transform of the editable text on the page.
-  EditableTextGeometry _geometry;
+  EditableTextGeometry? _geometry;
 
-  _OnChangeCallback _onChange;
-  _OnActionCallback _onAction;
+  _OnChangeCallback? _onChange;
+  _OnActionCallback? _onAction;
 
   final List<StreamSubscription<html.Event>> _subscriptions =
       <StreamSubscription<html.Event>>[];
 
   bool get hasAutofillGroup => _inputConfiguration.autofillGroup != null;
 
-  html.FormElement get focusedFormElement =>
-      _inputConfiguration.autofillGroup.formElement;
+  html.FormElement? get focusedFormElement =>
+      _inputConfiguration.autofillGroup?.formElement;
 
   @override
   void initializeTextEditing(
     InputConfiguration inputConfig, {
-    @required _OnChangeCallback onChange,
-    @required _OnActionCallback onAction,
+    required _OnChangeCallback onChange,
+    required _OnActionCallback onAction,
   }) {
     assert(!isEnabled);
 
     this._inputConfiguration = inputConfig;
 
-    domElement = inputConfig.inputType.createDomElement();
-    if (inputConfig.obscureText) {
+    _domElement = inputConfig.inputType.createDomElement();
+    if (inputConfig.obscureText!) {
       domElement.setAttribute('type', 'password');
     }
 
     inputConfig.autofill?.applyToDomElement(domElement, focusedElement: true);
 
-    final String autocorrectValue = inputConfig.autocorrect ? 'on' : 'off';
+    final String autocorrectValue = inputConfig.autocorrect! ? 'on' : 'off';
     domElement.setAttribute('autocorrect', autocorrectValue);
 
     _setStaticStyleAttributes(domElement);
@@ -598,7 +600,7 @@ abstract class DefaultTextEditingStrategy implements TextEditingStrategy {
       // DOM later, when the first location information arrived.
       // Otherwise, on Blink based Desktop browsers, the autofill menu appears
       // on top left of the screen.
-      domRenderer.glassPaneElement.append(domElement);
+      domRenderer.glassPaneElement!.append(domElement);
     }
 
     initializeElementPlacement();
@@ -617,7 +619,7 @@ abstract class DefaultTextEditingStrategy implements TextEditingStrategy {
   void addEventHandlers() {
     if (_inputConfiguration.autofillGroup != null) {
       _subscriptions
-          .addAll(_inputConfiguration.autofillGroup.addInputEventListeners());
+          .addAll(_inputConfiguration.autofillGroup!.addInputEventListeners());
     }
 
     // Subscribe to text and selection changes.
@@ -649,7 +651,7 @@ abstract class DefaultTextEditingStrategy implements TextEditingStrategy {
   void updateElementStyle(EditableTextStyle style) {
     _style = style;
     if (isEnabled) {
-      _style.applyToDomElement(domElement);
+      _style!.applyToDomElement(domElement);
     }
   }
 
@@ -667,18 +669,18 @@ abstract class DefaultTextEditingStrategy implements TextEditingStrategy {
     }
     _subscriptions.clear();
     domElement.remove();
-    domElement = null;
+    _domElement = null;
     _inputConfiguration.autofillGroup?.removeForm();
   }
 
   @mustCallSuper
   @override
-  void setEditingState(EditingState editingState) {
+  void setEditingState(EditingState? editingState) {
     _lastEditingState = editingState;
-    if (!isEnabled || !editingState.isValid) {
+    if (!isEnabled || !editingState!.isValid) {
       return;
     }
-    _lastEditingState.applyToDomElement(domElement);
+    _lastEditingState!.applyToDomElement(domElement);
   }
 
   void placeElement() {
@@ -686,20 +688,17 @@ abstract class DefaultTextEditingStrategy implements TextEditingStrategy {
   }
 
   void placeForm() {
-    _inputConfiguration.autofillGroup.placeForm(domElement);
+    _inputConfiguration.autofillGroup!.placeForm(domElement);
   }
 
   void _handleChange(html.Event event) {
     assert(isEnabled);
-    assert(domElement != null);
 
     EditingState newEditingState = EditingState.fromDomElement(domElement);
 
-    assert(newEditingState != null);
-
     if (newEditingState != _lastEditingState) {
       _lastEditingState = newEditingState;
-      _onChange(_lastEditingState);
+      _onChange!(_lastEditingState);
     }
   }
 
@@ -708,7 +707,7 @@ abstract class DefaultTextEditingStrategy implements TextEditingStrategy {
       if (_inputConfiguration.inputType.submitActionOnEnter &&
           event.keyCode == _kReturnKeyCode) {
         event.preventDefault();
-        _onAction(_inputConfiguration.inputAction);
+        _onAction!(_inputConfiguration.inputAction);
       }
     }
   }
@@ -723,8 +722,8 @@ abstract class DefaultTextEditingStrategy implements TextEditingStrategy {
   /// - Selection changes.
   void enable(
     InputConfiguration inputConfig, {
-    @required _OnChangeCallback onChange,
-    @required _OnActionCallback onAction,
+    required _OnChangeCallback onChange,
+    required _OnActionCallback onAction,
   }) {
     assert(!isEnabled);
 
@@ -788,7 +787,7 @@ class IOSTextEditingStrategy extends GloballyPositionedTextEditingStrategy {
   /// focus is called we are positioning it offscreen. The location of the input
   /// in iOS is set to correct place, 100ms after focus. We use this timer for
   /// timing this delay.
-  Timer _positionInputElementTimer;
+  Timer? _positionInputElementTimer;
   static const Duration _delayBeforePlacement =
       const Duration(milliseconds: 100);
 
@@ -808,8 +807,8 @@ class IOSTextEditingStrategy extends GloballyPositionedTextEditingStrategy {
   @override
   void initializeTextEditing(
     InputConfiguration inputConfig, {
-    @required _OnChangeCallback onChange,
-    @required _OnActionCallback onAction,
+    required _OnChangeCallback onChange,
+    required _OnActionCallback onAction,
   }) {
     super.initializeTextEditing(inputConfig,
         onChange: onChange, onAction: onAction);
@@ -817,7 +816,7 @@ class IOSTextEditingStrategy extends GloballyPositionedTextEditingStrategy {
     if (hasAutofillGroup) {
       placeForm();
     } else {
-      domRenderer.glassPaneElement.append(domElement);
+      domRenderer.glassPaneElement!.append(domElement);
     }
   }
 
@@ -835,7 +834,7 @@ class IOSTextEditingStrategy extends GloballyPositionedTextEditingStrategy {
   void addEventHandlers() {
     if (_inputConfiguration.autofillGroup != null) {
       _subscriptions
-          .addAll(_inputConfiguration.autofillGroup.addInputEventListeners());
+          .addAll(_inputConfiguration.autofillGroup!.addInputEventListeners());
     }
 
     // Subscribe to text and selection changes.
@@ -938,8 +937,8 @@ class AndroidTextEditingStrategy extends GloballyPositionedTextEditingStrategy {
   @override
   void initializeTextEditing(
     InputConfiguration inputConfig, {
-    @required _OnChangeCallback onChange,
-    @required _OnActionCallback onAction,
+    required _OnChangeCallback onChange,
+    required _OnActionCallback onAction,
   }) {
     super.initializeTextEditing(inputConfig,
         onChange: onChange, onAction: onAction);
@@ -947,7 +946,7 @@ class AndroidTextEditingStrategy extends GloballyPositionedTextEditingStrategy {
     if (hasAutofillGroup) {
       placeForm();
     } else {
-      domRenderer.glassPaneElement.append(domElement);
+      domRenderer.glassPaneElement!.append(domElement);
     }
   }
 
@@ -955,7 +954,7 @@ class AndroidTextEditingStrategy extends GloballyPositionedTextEditingStrategy {
   void addEventHandlers() {
     if (_inputConfiguration.autofillGroup != null) {
       _subscriptions
-          .addAll(_inputConfiguration.autofillGroup.addInputEventListeners());
+          .addAll(_inputConfiguration.autofillGroup!.addInputEventListeners());
     }
 
     // Subscribe to text and selection changes.
@@ -966,7 +965,7 @@ class AndroidTextEditingStrategy extends GloballyPositionedTextEditingStrategy {
     _subscriptions.add(html.document.onSelectionChange.listen(_handleChange));
 
     _subscriptions.add(domElement.onBlur.listen((_) {
-      if (domRenderer.windowHasFocus) {
+      if (domRenderer.windowHasFocus!) {
         // Chrome on Android will hide the onscreen keyboard when you tap outside
         // the text box. Instead, we want the framework to tell us to hide the
         // keyboard via `TextInput.clearClient` or `TextInput.hide`. Therefore
@@ -995,15 +994,15 @@ class FirefoxTextEditingStrategy extends GloballyPositionedTextEditingStrategy {
   @override
   void initializeTextEditing(
     InputConfiguration inputConfig, {
-    @required _OnChangeCallback onChange,
-    @required _OnActionCallback onAction,
+    required _OnChangeCallback onChange,
+    required _OnActionCallback onAction,
   }) {
     super.initializeTextEditing(inputConfig,
         onChange: onChange, onAction: onAction);
     if (hasAutofillGroup) {
       placeForm();
     } else {
-      domRenderer.glassPaneElement.append(domElement);
+      domRenderer.glassPaneElement!.append(domElement);
     }
   }
 
@@ -1011,7 +1010,7 @@ class FirefoxTextEditingStrategy extends GloballyPositionedTextEditingStrategy {
   void addEventHandlers() {
     if (_inputConfiguration.autofillGroup != null) {
       _subscriptions
-          .addAll(_inputConfiguration.autofillGroup.addInputEventListeners());
+          .addAll(_inputConfiguration.autofillGroup!.addInputEventListeners());
     }
 
     // Subscribe to text and selection changes.
@@ -1069,7 +1068,7 @@ class TextEditingChannel {
 
   /// Handles "flutter/textinput" platform messages received from the framework.
   void handleTextInput(
-      ByteData data, ui.PlatformMessageResponseCallback callback) {
+      ByteData? data, ui.PlatformMessageResponseCallback? callback) {
     const JSONMethodCodec codec = JSONMethodCodec();
     final MethodCall call = codec.decodeMethodCall(data);
     switch (call.method) {
@@ -1120,14 +1119,14 @@ class TextEditingChannel {
   }
 
   /// Sends the 'TextInputClient.updateEditingState' message to the framework.
-  void updateEditingState(int clientId, EditingState editingState) {
+  void updateEditingState(int? clientId, EditingState? editingState) {
     if (window._onPlatformMessage != null) {
       window.invokeOnPlatformMessage(
         'flutter/textinput',
         const JSONMethodCodec().encodeMethodCall(
           MethodCall('TextInputClient.updateEditingState', <dynamic>[
             clientId,
-            editingState.toFlutter(),
+            editingState!.toFlutter(),
           ]),
         ),
         _emptyCallback,
@@ -1136,7 +1135,7 @@ class TextEditingChannel {
   }
 
   /// Sends the 'TextInputClient.performAction' message to the framework.
-  void performAction(int clientId, String inputAction) {
+  void performAction(int? clientId, String? inputAction) {
     if (window._onPlatformMessage != null) {
       window.invokeOnPlatformMessage(
         'flutter/textinput',
@@ -1152,7 +1151,7 @@ class TextEditingChannel {
   }
 
   /// Sends the 'TextInputClient.onConnectionClosed' message to the framework.
-  void onConnectionClosed(int clientId) {
+  void onConnectionClosed(int? clientId) {
     if (window._onPlatformMessage != null) {
       window.invokeOnPlatformMessage(
         'flutter/textinput',
@@ -1199,22 +1198,22 @@ class HybridTextEditing {
     channel = TextEditingChannel(this);
   }
 
-  TextEditingChannel channel;
+  late TextEditingChannel channel;
 
   /// The text editing stategy used. It can change depending on the
   /// formfactor/browser.
   ///
   /// It uses an HTML element to manage editing state when a custom element is
   /// not provided via [useCustomEditableElement]
-  DefaultTextEditingStrategy _defaultEditingElement;
+  DefaultTextEditingStrategy? _defaultEditingElement;
 
   /// The HTML element used to manage editing state.
   ///
   /// This field is populated using [useCustomEditableElement]. If `null` the
   /// [_defaultEditingElement] is used instead.
-  DefaultTextEditingStrategy _customEditingElement;
+  DefaultTextEditingStrategy? _customEditingElement;
 
-  DefaultTextEditingStrategy get editingElement {
+  DefaultTextEditingStrategy? get editingElement {
     if (_customEditingElement != null) {
       return _customEditingElement;
     }
@@ -1222,7 +1221,7 @@ class HybridTextEditing {
   }
 
   /// Responds to the 'TextInput.setClient' message.
-  void setClient(int clientId, InputConfiguration configuration) {
+  void setClient(int? clientId, InputConfiguration configuration) {
     final bool clientIdChanged = _clientId != null && _clientId != clientId;
     if (clientIdChanged && isEditing) {
       stopEditing();
@@ -1233,7 +1232,7 @@ class HybridTextEditing {
 
   /// Responds to the 'TextInput.setEditingState' message.
   void setEditingState(EditingState state) {
-    editingElement.setEditingState(state);
+    editingElement!.setEditingState(state);
   }
 
   /// Responds to the 'TextInput.show' message.
@@ -1245,12 +1244,12 @@ class HybridTextEditing {
 
   /// Responds to the 'TextInput.setEditableSizeAndTransform' message.
   void setEditableSizeAndTransform(EditableTextGeometry geometry) {
-    editingElement.updateElementPlacement(geometry);
+    editingElement!.updateElementPlacement(geometry);
   }
 
   /// Responds to the 'TextInput.setStyle' message.
   void setStyle(EditableTextStyle style) {
-    editingElement.updateElementStyle(style);
+    editingElement!.updateElementStyle(style);
   }
 
   /// Responds to the 'TextInput.clearClient' message.
@@ -1279,7 +1278,7 @@ class HybridTextEditing {
   ///
   /// Use [stopUsingCustomEditableElement] to switch back to default element.
   void useCustomEditableElement(
-      DefaultTextEditingStrategy customEditingElement) {
+      DefaultTextEditingStrategy? customEditingElement) {
     if (isEditing && customEditingElement != _customEditingElement) {
       stopEditing();
     }
@@ -1292,7 +1291,7 @@ class HybridTextEditing {
     useCustomEditableElement(null);
   }
 
-  int _clientId;
+  int? _clientId;
 
   /// Flag which shows if there is an ongoing editing.
   ///
@@ -1300,17 +1299,17 @@ class HybridTextEditing {
   @visibleForTesting
   bool isEditing = false;
 
-  InputConfiguration _configuration;
+  late InputConfiguration _configuration;
 
   void _startEditing() {
     assert(!isEditing);
     isEditing = true;
-    editingElement.enable(
+    editingElement!.enable(
       _configuration,
-      onChange: (EditingState editingState) {
+      onChange: (EditingState? editingState) {
         channel.updateEditingState(_clientId, editingState);
       },
-      onAction: (String inputAction) {
+      onAction: (String? inputAction) {
         channel.performAction(_clientId, inputAction);
       },
     );
@@ -1319,7 +1318,7 @@ class HybridTextEditing {
   void stopEditing() {
     assert(isEditing);
     isEditing = false;
-    editingElement.disable();
+    editingElement!.disable();
   }
 
   void sendTextConnectionClosedToFrameworkIfAny() {
@@ -1335,11 +1334,11 @@ class HybridTextEditing {
 /// This information is received via TextInput.setStyle message.
 class EditableTextStyle {
   EditableTextStyle({
-    @required this.textDirection,
-    @required this.fontSize,
-    @required this.textAlign,
-    @required this.fontFamily,
-    @required this.fontWeight,
+    required this.textDirection,
+    required this.fontSize,
+    required this.textAlign,
+    required this.fontFamily,
+    required this.fontWeight,
   });
 
   factory EditableTextStyle.fromFrameworkMessage(
@@ -1351,7 +1350,7 @@ class EditableTextStyle {
 
     final int textAlignIndex = flutterStyle['textAlignIndex'];
     final int textDirectionIndex = flutterStyle['textDirectionIndex'];
-    final int fontWeightIndex = flutterStyle['fontWeightIndex'];
+    final int? fontWeightIndex = flutterStyle['fontWeightIndex'];
 
     // Convert [fontWeightIndex] to its CSS equivalent value.
     final String fontWeight = fontWeightIndex != null
@@ -1372,13 +1371,13 @@ class EditableTextStyle {
 
   /// This information will be used for changing the style of the hidden input
   /// element, which will match it's size to the size of the editable widget.
-  final double fontSize;
+  final double? fontSize;
   final String fontWeight;
-  final String fontFamily;
+  final String? fontFamily;
   final ui.TextAlign textAlign;
   final ui.TextDirection textDirection;
 
-  String get align => textAlignToCssValue(textAlign, textDirection);
+  String? get align => textAlignToCssValue(textAlign, textDirection);
 
   String get cssFont => '${fontWeight} ${fontSize}px ${fontFamily}';
 
@@ -1396,9 +1395,9 @@ class EditableTextStyle {
 @immutable
 class EditableTextGeometry {
   EditableTextGeometry({
-    @required this.width,
-    @required this.height,
-    @required this.globalTransform,
+    required this.width,
+    required this.height,
+    required this.globalTransform,
   });
 
   /// Parses the geometry from a message sent by the framework.
diff --git a/lib/web_ui/lib/src/engine/util.dart b/lib/web_ui/lib/src/engine/util.dart
index 1f4e5c386aab5..67f991eccadb2 100644
--- a/lib/web_ui/lib/src/engine/util.dart
+++ b/lib/web_ui/lib/src/engine/util.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// Generic callback signature, used by [_futurize].
@@ -12,7 +12,7 @@ typedef Callback<T> = void Function(T result);
 ///
 /// Return value should be null on success, and a string error message on
 /// failure.
-typedef Callbacker<T> = String/*?*/ Function(Callback<T> callback);
+typedef Callbacker<T> = String? Function(Callback<T> callback);
 
 /// Converts a method that receives a value-returning callback to a method that
 /// returns a Future.
@@ -35,9 +35,9 @@ typedef Callbacker<T> = String/*?*/ Function(Callback<T> callback);
 ///   return _futurize(_doSomethingAndCallback);
 /// }
 /// ```
-Future<T>/*!*/ futurize<T>(Callbacker<T> callbacker) {
+Future<T> futurize<T>(Callbacker<T> callbacker) {
   final Completer<T> completer = Completer<T>.sync();
-  final String error = callbacker((T t) {
+  final String? error = callbacker((T t) {
     if (t == null) {
       completer.completeError(Exception('operation failed'));
     } else {
@@ -319,13 +319,13 @@ String _pathToSvgClipPath(ui.Path path,
   sb.write('<clipPath id=$clipId clipPathUnits="objectBoundingBox">');
 
   sb.write('<path transform="scale($scaleX, $scaleY)" fill="#FFFFFF" d="');
-  pathToSvg(path, sb, offsetX: offsetX, offsetY: offsetY);
+  pathToSvg(path as SurfacePath, sb, offsetX: offsetX, offsetY: offsetY);
   sb.write('"></path></clipPath></defs></svg');
   return sb.toString();
 }
 
 /// Converts color to a css compatible attribute value.
-String colorToCssString(ui.Color color) {
+String? colorToCssString(ui.Color? color) {
   if (color == null) {
     return null;
   }
@@ -430,7 +430,7 @@ bool get _isMacOrIOS => operatingSystem == OperatingSystem.iOs ||
 ///
 /// If the given [fontFamily] is a generic font-family, then just return it.
 /// Otherwise, wrap the family name in quotes and add a fallback font family.
-String canonicalizeFontFamily(String fontFamily) {
+String? canonicalizeFontFamily(String? fontFamily) {
   if (_genericFontFamilies.contains(fontFamily)) {
     return fontFamily;
   }
@@ -449,9 +449,6 @@ String canonicalizeFontFamily(String fontFamily) {
 
 /// Converts a list of [Offset] to a typed array of floats.
 Float32List offsetListToFloat32List(List<ui.Offset> offsetList) {
-  if (offsetList == null) {
-    return null;
-  }
   final int length = offsetList.length;
   final floatList = Float32List(length * 2);
   for (int i = 0, destIndex = 0; i < length; i++, destIndex += 2) {
@@ -471,13 +468,13 @@ Float32List offsetListToFloat32List(List<ui.Offset> offsetList) {
 ///
 /// * Use 3D transform instead of 2D: this does not work because it causes text
 ///   blurriness: https://github.com/flutter/flutter/issues/32274
-void applyWebkitClipFix(html.Element containerElement) {
+void applyWebkitClipFix(html.Element? containerElement) {
   if (browserEngine == BrowserEngine.webkit) {
-    containerElement.style.zIndex = '0';
+    containerElement!.style.zIndex = '0';
   }
 }
 
-final ByteData _fontChangeMessage = JSONMessageCodec().encodeMessage(<String, dynamic>{'type': 'fontsChange'});
+final ByteData? _fontChangeMessage = JSONMessageCodec().encodeMessage(<String, dynamic>{'type': 'fontsChange'});
 
 // Font load callbacks will typically arrive in sequence, we want to prevent
 // sendFontChangeMessage of causing multiple synchronous rebuilds.
@@ -520,3 +517,13 @@ class _FastMatrix64 {
 double convertSigmaToRadius(double sigma) {
   return sigma * 2.0;
 }
+
+/// Used to check for null values that are non-nullable.
+///
+/// This is useful when some external API (e.g. HTML DOM) disagrees with
+/// Dart type declarations (e.g. `dart:html`). Where `dart:html` may believe
+/// something to be non-null, it may actually be null (e.g. old browsers do
+/// not implement a feature, such as clipboard).
+bool isUnsoundNull(dynamic object) {
+  return object == null;
+}
diff --git a/lib/web_ui/lib/src/engine/validators.dart b/lib/web_ui/lib/src/engine/validators.dart
index ec1c0bb5a999c..243e1326c9009 100644
--- a/lib/web_ui/lib/src/engine/validators.dart
+++ b/lib/web_ui/lib/src/engine/validators.dart
@@ -2,11 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 bool rectIsValid(ui.Rect rect) {
-  assert(rect != null, 'Rect argument was null.');
+  assert(rect != null, 'Rect argument was null.'); // ignore: unnecessary_null_comparison
   assert(
       !(rect.left.isNaN ||
           rect.right.isNaN ||
@@ -17,7 +17,7 @@ bool rectIsValid(ui.Rect rect) {
 }
 
 bool rrectIsValid(ui.RRect rrect) {
-  assert(rrect != null, 'RRect argument was null.');
+  assert(rrect != null, 'RRect argument was null.'); // ignore: unnecessary_null_comparison
   assert(
       !(rrect.left.isNaN ||
           rrect.right.isNaN ||
@@ -28,20 +28,20 @@ bool rrectIsValid(ui.RRect rrect) {
 }
 
 bool offsetIsValid(ui.Offset offset) {
-  assert(offset != null, 'Offset argument was null.');
+  assert(offset != null, 'Offset argument was null.'); // ignore: unnecessary_null_comparison
   assert(!offset.dx.isNaN && !offset.dy.isNaN,
       'Offset argument contained a NaN value.');
   return true;
 }
 
 bool matrix4IsValid(Float32List matrix4) {
-  assert(matrix4 != null, 'Matrix4 argument was null.');
+  assert(matrix4 != null, 'Matrix4 argument was null.'); // ignore: unnecessary_null_comparison
   assert(matrix4.length == 16, 'Matrix4 must have 16 entries.');
   return true;
 }
 
 bool radiusIsValid(ui.Radius radius) {
-  assert(radius != null, 'Radius argument was null.');
+  assert(radius != null, 'Radius argument was null.'); // ignore: unnecessary_null_comparison
   assert(!radius.x.isNaN && !radius.y.isNaN,
       'Radius argument contained a NaN value.');
   return true;
diff --git a/lib/web_ui/lib/src/engine/vector_math.dart b/lib/web_ui/lib/src/engine/vector_math.dart
index 721e4ed3c9004..9dfe164091605 100644
--- a/lib/web_ui/lib/src/engine/vector_math.dart
+++ b/lib/web_ui/lib/src/engine/vector_math.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
 class Matrix4 {
@@ -13,7 +12,7 @@ class Matrix4 {
 
   /// Returns a matrix that is the inverse of [other] if [other] is invertible,
   /// otherwise `null`.
-  static Matrix4 tryInvert(Matrix4 other) {
+  static Matrix4? tryInvert(Matrix4 other) {
     final Matrix4 r = Matrix4.zero();
     final double determinant = r.copyInverse(other);
     if (determinant == 0.0) {
@@ -243,7 +242,7 @@ class Matrix4 {
   }
 
   /// Scale this matrix by a [Vector3], [Vector4], or x,y,z
-  void scale(double x, [double y, double z]) {
+  void scale(double x, [double? y, double? z]) {
     final double sx = x;
     final double sy = y ?? x;
     final double sz = z ?? x;
@@ -268,7 +267,7 @@ class Matrix4 {
 
   /// Create a copy of [this] scaled by a [Vector3], [Vector4] or [x],[y], and
   /// [z].
-  Matrix4 scaled(double x, [double y, double z]) => clone()..scale(x, y, z);
+  Matrix4 scaled(double x, [double? y, double? z]) => clone()..scale(x, y, z);
 
   /// Zeros [this].
   void setZero() {
@@ -963,7 +962,7 @@ class Matrix4 {
 
   /// Rotate a copy of [arg] of type [Vector3] using the rotation defined by
   /// [this]. If a [out] parameter is supplied, the copy is stored in [out].
-  Vector3 rotated3(Vector3 arg, [Vector3 out]) {
+  Vector3 rotated3(Vector3 arg, [Vector3? out]) {
     if (out == null) {
       out = Vector3.copy(arg);
     } else {
@@ -1132,7 +1131,7 @@ class Vector3 {
 
   /// Generate random vector in the range (0, 0, 0) to (1, 1, 1). You can
   /// optionally pass your own random number generator.
-  factory Vector3.random([math.Random rng]) {
+  factory Vector3.random([math.Random? rng]) {
     rng ??= math.Random();
     return Vector3(rng.nextDouble(), rng.nextDouble(), rng.nextDouble());
   }
diff --git a/lib/web_ui/lib/src/engine/web_experiments.dart b/lib/web_ui/lib/src/engine/web_experiments.dart
index 2e2050938e8df..34e7047e21e27 100644
--- a/lib/web_ui/lib/src/engine/web_experiments.dart
+++ b/lib/web_ui/lib/src/engine/web_experiments.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
 part of engine;
 
 /// A bag of all experiment flags in the web engine.
@@ -18,35 +17,34 @@ class WebExperiments {
   }
 
   static WebExperiments ensureInitialized() {
-    if (WebExperiments.instance == null) {
-      WebExperiments.instance = WebExperiments._();
-    }
-    return WebExperiments.instance;
+    return WebExperiments.instance ?? (WebExperiments.instance = WebExperiments._());
   }
 
-  static WebExperiments instance;
+  static WebExperiments? instance;
 
   /// Experiment flag for using canvas-based text measurement.
-  bool get useCanvasText => _useCanvasText ?? false;
-  set useCanvasText(bool enabled) {
-    _useCanvasText = enabled;
+  bool get useCanvasText => _useCanvasText;
+  set useCanvasText(bool? enabled) {
+    _useCanvasText = enabled ?? _defaultUseCanvasText;
   }
 
-  bool _useCanvasText = const bool.fromEnvironment(
+  static const bool _defaultUseCanvasText = const bool.fromEnvironment(
     'FLUTTER_WEB_USE_EXPERIMENTAL_CANVAS_TEXT',
-    defaultValue: null,
+    defaultValue: false,
   );
 
+  bool _useCanvasText = _defaultUseCanvasText;
+
   /// Reset all experimental flags to their default values.
   void reset() {
-    _useCanvasText = null;
+    _useCanvasText = _defaultUseCanvasText;
   }
 
   /// Used to enable/disable experimental flags in the web engine.
   void updateExperiment(String name, bool enabled) {
     switch (name) {
       case 'useCanvasText':
-        _useCanvasText = enabled;
+        useCanvasText = enabled;
         break;
     }
   }
diff --git a/lib/web_ui/lib/src/engine/window.dart b/lib/web_ui/lib/src/engine/window.dart
index dc764de1e76ee..9c76e50c80dba 100644
--- a/lib/web_ui/lib/src/engine/window.dart
+++ b/lib/web_ui/lib/src/engine/window.dart
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+
 part of engine;
 
 /// When set to true, all platform messages will be printed to the console.
@@ -11,7 +11,7 @@ const bool _debugPrintPlatformMessages = false;
 /// Requests that the browser schedule a frame.
 ///
 /// This may be overridden in tests, for example, to pump fake frames.
-ui.VoidCallback scheduleFrameCallback;
+ui.VoidCallback? scheduleFrameCallback;
 
 /// The Web implementation of [ui.Window].
 class EngineWindow extends ui.Window {
@@ -26,14 +26,12 @@ class EngineWindow extends ui.Window {
   }
 
   @override
-  double get devicePixelRatio => _debugDevicePixelRatio != null
-      ? _debugDevicePixelRatio
-      : browserDevicePixelRatio;
+  double get devicePixelRatio => _debugDevicePixelRatio ?? browserDevicePixelRatio;
 
   /// Returns device pixel ratio returned by browser.
   static double get browserDevicePixelRatio {
-    double ratio = html.window.devicePixelRatio;
-    // Guard against WebOS returning 0.
+    double? ratio = html.window.devicePixelRatio as double?;
+    // Guard against WebOS returning 0 and other browsers returning null.
     return (ratio == null || ratio == 0.0) ? 1.0 : ratio;
   }
 
@@ -44,7 +42,7 @@ class EngineWindow extends ui.Window {
     _debugDevicePixelRatio = value;
   }
 
-  double _debugDevicePixelRatio;
+  double? _debugDevicePixelRatio;
 
   @override
   ui.Size get physicalSize {
@@ -52,7 +50,7 @@ class EngineWindow extends ui.Window {
       _computePhysicalSize();
     }
     assert(_physicalSize != null);
-    return _physicalSize;
+    return _physicalSize!;
   }
 
   /// Computes the physical size of the screen from [html.window].
@@ -73,10 +71,10 @@ class EngineWindow extends ui.Window {
     if (!override) {
       double windowInnerWidth;
       double windowInnerHeight;
-      final html.VisualViewport viewport = html.window.visualViewport;
+      final html.VisualViewport? viewport = html.window.visualViewport;
       if (viewport != null) {
-        windowInnerWidth = viewport.width * devicePixelRatio;
-        windowInnerHeight = viewport.height * devicePixelRatio;
+        windowInnerWidth = viewport.width * devicePixelRatio as double;
+        windowInnerHeight = viewport.height * devicePixelRatio as double;
       } else {
         windowInnerWidth = html.window.innerWidth * devicePixelRatio;
         windowInnerHeight = html.window.innerHeight * devicePixelRatio;
@@ -90,13 +88,13 @@ class EngineWindow extends ui.Window {
 
   void computeOnScreenKeyboardInsets() {
     double windowInnerHeight;
-    final html.VisualViewport viewport = html.window.visualViewport;
+    final html.VisualViewport? viewport = html.window.visualViewport;
     if (viewport != null) {
-      windowInnerHeight = viewport.height * devicePixelRatio;
+      windowInnerHeight = viewport.height * devicePixelRatio as double;
     } else {
       windowInnerHeight = html.window.innerHeight * devicePixelRatio;
     }
-    final double bottomPadding = _physicalSize.height - windowInnerHeight;
+    final double bottomPadding = _physicalSize!.height - windowInnerHeight;
     _viewInsets =
         WindowPadding(bottom: bottomPadding, left: 0, right: 0, top: 0);
   }
@@ -119,18 +117,18 @@ class EngineWindow extends ui.Window {
     double height = 0;
     double width = 0;
     if (html.window.visualViewport != null) {
-      height = html.window.visualViewport.height * devicePixelRatio;
-      width = html.window.visualViewport.width * devicePixelRatio;
+      height = html.window.visualViewport!.height * devicePixelRatio as double;
+      width = html.window.visualViewport!.width * devicePixelRatio as double;
     } else {
       height = html.window.innerHeight * devicePixelRatio;
       width = html.window.innerWidth * devicePixelRatio;
     }
     // First confirm both heught and width is effected.
-    if (_physicalSize.height != height && _physicalSize.width != width) {
+    if (_physicalSize!.height != height && _physicalSize!.width != width) {
       // If prior to rotation height is bigger than width it should be the
       // opposite after the rotation and vice versa.
-      if ((_physicalSize.height > _physicalSize.width && height < width) ||
-          (_physicalSize.width > _physicalSize.height && width < height)) {
+      if ((_physicalSize!.height > _physicalSize!.width && height < width) ||
+          (_physicalSize!.width > _physicalSize!.height && width < height)) {
         // Rotation detected
         return true;
       }
@@ -140,13 +138,13 @@ class EngineWindow extends ui.Window {
 
   @override
   WindowPadding get viewInsets => _viewInsets;
-  WindowPadding _viewInsets = ui.WindowPadding.zero;
+  WindowPadding _viewInsets = ui.WindowPadding.zero as WindowPadding;
 
   /// Lazily populated and cleared at the end of the frame.
-  ui.Size _physicalSize;
+  ui.Size? _physicalSize;
 
   /// Overrides the value of [physicalSize] in tests.
-  ui.Size webOnlyDebugPhysicalSizeOverride;
+  ui.Size? webOnlyDebugPhysicalSizeOverride;
 
   @override
   double get physicalDepth => double.maxFinite;
@@ -162,10 +160,10 @@ class EngineWindow extends ui.Window {
   ///
   /// The reason for the lazy initialization is to give enough time for the app to set [locationStrategy]
   /// in `lib/src/ui/initialization.dart`.
-  String _defaultRouteName;
+  String? _defaultRouteName;
 
   @override
-  String/*!*/ get defaultRouteName => _defaultRouteName ??= _browserHistory.currentPath;
+  String get defaultRouteName => _defaultRouteName ??= _browserHistory.currentPath;
 
   @override
   void scheduleFrame() {
@@ -173,27 +171,27 @@ class EngineWindow extends ui.Window {
       throw new Exception(
           'scheduleFrameCallback must be initialized first.');
     }
-    scheduleFrameCallback();
+    scheduleFrameCallback!();
   }
 
   /// Change the strategy to use for handling browser history location.
   /// Setting this member will automatically update [_browserHistory].
   ///
   /// By setting this to null, the browser history will be disabled.
-  set locationStrategy(LocationStrategy strategy) {
+  set locationStrategy(LocationStrategy? strategy) {
     _browserHistory.locationStrategy = strategy;
   }
 
   /// Returns the currently active location strategy.
   @visibleForTesting
-  LocationStrategy get locationStrategy => _browserHistory.locationStrategy;
+  LocationStrategy? get locationStrategy => _browserHistory.locationStrategy;
 
   @override
-  ui.VoidCallback get onTextScaleFactorChanged => _onTextScaleFactorChanged;
-  ui.VoidCallback _onTextScaleFactorChanged;
-  Zone _onTextScaleFactorChangedZone;
+  ui.VoidCallback? get onTextScaleFactorChanged => _onTextScaleFactorChanged;
+  ui.VoidCallback? _onTextScaleFactorChanged;
+  Zone? _onTextScaleFactorChangedZone;
   @override
-  set onTextScaleFactorChanged(ui.VoidCallback callback) {
+  set onTextScaleFactorChanged(ui.VoidCallback? callback) {
     _onTextScaleFactorChanged = callback;
     _onTextScaleFactorChangedZone = Zone.current;
   }
@@ -205,12 +203,12 @@ class EngineWindow extends ui.Window {
   }
 
   @override
-  ui.VoidCallback get onPlatformBrightnessChanged =>
+  ui.VoidCallback? get onPlatformBrightnessChanged =>
       _onPlatformBrightnessChanged;
-  ui.VoidCallback _onPlatformBrightnessChanged;
-  Zone _onPlatformBrightnessChangedZone;
+  ui.VoidCallback? _onPlatformBrightnessChanged;
+  Zone? _onPlatformBrightnessChangedZone;
   @override
-  set onPlatformBrightnessChanged(ui.VoidCallback callback) {
+  set onPlatformBrightnessChanged(ui.VoidCallback? callback) {
     _onPlatformBrightnessChanged = callback;
     _onPlatformBrightnessChangedZone = Zone.current;
   }
@@ -222,11 +220,11 @@ class EngineWindow extends ui.Window {
   }
 
   @override
-  ui.VoidCallback/*?*/ get onMetricsChanged => _onMetricsChanged;
-  ui.VoidCallback/*?*/ _onMetricsChanged;
-  Zone/*!*/ _onMetricsChangedZone = Zone.root;
+  ui.VoidCallback? get onMetricsChanged => _onMetricsChanged;
+  ui.VoidCallback? _onMetricsChanged;
+  Zone _onMetricsChangedZone = Zone.root;
   @override
-  set onMetricsChanged(ui.VoidCallback callback) {
+  set onMetricsChanged(ui.VoidCallback? callback) {
     _onMetricsChanged = callback;
     _onMetricsChangedZone = Zone.current;
   }
@@ -240,11 +238,11 @@ class EngineWindow extends ui.Window {
   }
 
   @override
-  ui.VoidCallback get onLocaleChanged => _onLocaleChanged;
-  ui.VoidCallback _onLocaleChanged;
-  Zone _onLocaleChangedZone;
+  ui.VoidCallback? get onLocaleChanged => _onLocaleChanged;
+  ui.VoidCallback? _onLocaleChanged;
+  Zone? _onLocaleChangedZone;
   @override
-  set onLocaleChanged(ui.VoidCallback callback) {
+  set onLocaleChanged(ui.VoidCallback? callback) {
     _onLocaleChanged = callback;
     _onLocaleChangedZone = Zone.current;
   }
@@ -261,11 +259,11 @@ class EngineWindow extends ui.Window {
   /// * https://developer.mozilla.org/en-US/docs/Web/API/NavigatorLanguage/languages,
   ///   which explains browser quirks in the implementation notes.
   @override
-  ui.Locale get locale => _locales.first;
+  ui.Locale get locale => _locales!.first;
 
   @override
-  List<ui.Locale> get locales => _locales;
-  List<ui.Locale> _locales = parseBrowserLanguages();
+  List<ui.Locale>? get locales => _locales;
+  List<ui.Locale>? _locales = parseBrowserLanguages();
 
   /// Sets locales to `null`.
   ///
@@ -310,11 +308,11 @@ class EngineWindow extends ui.Window {
   }
 
   @override
-  ui.FrameCallback get onBeginFrame => _onBeginFrame;
-  ui.FrameCallback _onBeginFrame;
-  Zone _onBeginFrameZone;
+  ui.FrameCallback? get onBeginFrame => _onBeginFrame;
+  ui.FrameCallback? _onBeginFrame;
+  Zone? _onBeginFrameZone;
   @override
-  set onBeginFrame(ui.FrameCallback callback) {
+  set onBeginFrame(ui.FrameCallback? callback) {
     _onBeginFrame = callback;
     _onBeginFrameZone = Zone.current;
   }
@@ -326,11 +324,11 @@ class EngineWindow extends ui.Window {
   }
 
   @override
-  ui.TimingsCallback get onReportTimings => _onReportTimings;
-  ui.TimingsCallback _onReportTimings;
-  Zone _onReportTimingsZone;
+  ui.TimingsCallback? get onReportTimings => _onReportTimings;
+  ui.TimingsCallback? _onReportTimings;
+  Zone? _onReportTimingsZone;
   @override
-  set onReportTimings(ui.TimingsCallback callback) {
+  set onReportTimings(ui.TimingsCallback? callback) {
     _onReportTimings = callback;
     _onReportTimingsZone = Zone.current;
   }
@@ -343,11 +341,11 @@ class EngineWindow extends ui.Window {
   }
 
   @override
-  ui.VoidCallback get onDrawFrame => _onDrawFrame;
-  ui.VoidCallback _onDrawFrame;
-  Zone _onDrawFrameZone;
+  ui.VoidCallback? get onDrawFrame => _onDrawFrame;
+  ui.VoidCallback? _onDrawFrame;
+  Zone? _onDrawFrameZone;
   @override
-  set onDrawFrame(ui.VoidCallback callback) {
+  set onDrawFrame(ui.VoidCallback? callback) {
     _onDrawFrame = callback;
     _onDrawFrameZone = Zone.current;
   }
@@ -359,11 +357,11 @@ class EngineWindow extends ui.Window {
   }
 
   @override
-  ui.PointerDataPacketCallback get onPointerDataPacket => _onPointerDataPacket;
-  ui.PointerDataPacketCallback _onPointerDataPacket;
-  Zone _onPointerDataPacketZone;
+  ui.PointerDataPacketCallback? get onPointerDataPacket => _onPointerDataPacket;
+  ui.PointerDataPacketCallback? _onPointerDataPacket;
+  Zone? _onPointerDataPacketZone;
   @override
-  set onPointerDataPacket(ui.PointerDataPacketCallback callback) {
+  set onPointerDataPacket(ui.PointerDataPacketCallback? callback) {
     _onPointerDataPacket = callback;
     _onPointerDataPacketZone = Zone.current;
   }
@@ -376,11 +374,11 @@ class EngineWindow extends ui.Window {
   }
 
   @override
-  ui.VoidCallback get onSemanticsEnabledChanged => _onSemanticsEnabledChanged;
-  ui.VoidCallback _onSemanticsEnabledChanged;
-  Zone _onSemanticsEnabledChangedZone;
+  ui.VoidCallback? get onSemanticsEnabledChanged => _onSemanticsEnabledChanged;
+  ui.VoidCallback? _onSemanticsEnabledChanged;
+  Zone? _onSemanticsEnabledChangedZone;
   @override
-  set onSemanticsEnabledChanged(ui.VoidCallback callback) {
+  set onSemanticsEnabledChanged(ui.VoidCallback? callback) {
     _onSemanticsEnabledChanged = callback;
     _onSemanticsEnabledChangedZone = Zone.current;
   }
@@ -392,11 +390,11 @@ class EngineWindow extends ui.Window {
   }
 
   @override
-  ui.SemanticsActionCallback get onSemanticsAction => _onSemanticsAction;
-  ui.SemanticsActionCallback _onSemanticsAction;
-  Zone _onSemanticsActionZone;
+  ui.SemanticsActionCallback? get onSemanticsAction => _onSemanticsAction;
+  ui.SemanticsActionCallback? _onSemanticsAction;
+  Zone? _onSemanticsActionZone;
   @override
-  set onSemanticsAction(ui.SemanticsActionCallback callback) {
+  set onSemanticsAction(ui.SemanticsActionCallback? callback) {
     _onSemanticsAction = callback;
     _onSemanticsActionZone = Zone.current;
   }
@@ -404,18 +402,18 @@ class EngineWindow extends ui.Window {
   /// Engine code should use this method instead of the callback directly.
   /// Otherwise zones won't work properly.
   void invokeOnSemanticsAction(
-      int id, ui.SemanticsAction action, ByteData args) {
-    _invoke3<int, ui.SemanticsAction, ByteData>(
+      int id, ui.SemanticsAction action, ByteData? args) {
+    _invoke3<int, ui.SemanticsAction, ByteData?>(
         _onSemanticsAction, _onSemanticsActionZone, id, action, args);
   }
 
   @override
-  ui.VoidCallback get onAccessibilityFeaturesChanged =>
+  ui.VoidCallback? get onAccessibilityFeaturesChanged =>
       _onAccessibilityFeaturesChanged;
-  ui.VoidCallback _onAccessibilityFeaturesChanged;
-  Zone _onAccessibilityFeaturesChangedZone;
+  ui.VoidCallback? _onAccessibilityFeaturesChanged;
+  Zone? _onAccessibilityFeaturesChangedZone;
   @override
-  set onAccessibilityFeaturesChanged(ui.VoidCallback callback) {
+  set onAccessibilityFeaturesChanged(ui.VoidCallback? callback) {
     _onAccessibilityFeaturesChanged = callback;
     _onAccessibilityFeaturesChangedZone = Zone.current;
   }
@@ -428,11 +426,11 @@ class EngineWindow extends ui.Window {
   }
 
   @override
-  ui.PlatformMessageCallback get onPlatformMessage => _onPlatformMessage;
-  ui.PlatformMessageCallback _onPlatformMessage;
-  Zone _onPlatformMessageZone;
+  ui.PlatformMessageCallback? get onPlatformMessage => _onPlatformMessage;
+  ui.PlatformMessageCallback? _onPlatformMessage;
+  Zone? _onPlatformMessageZone;
   @override
-  set onPlatformMessage(ui.PlatformMessageCallback callback) {
+  set onPlatformMessage(ui.PlatformMessageCallback? callback) {
     _onPlatformMessage = callback;
     _onPlatformMessageZone = Zone.current;
   }
@@ -440,8 +438,8 @@ class EngineWindow extends ui.Window {
   /// Engine code should use this method instead of the callback directly.
   /// Otherwise zones won't work properly.
   void invokeOnPlatformMessage(
-      String name, ByteData data, ui.PlatformMessageResponseCallback callback) {
-    _invoke3<String, ByteData, ui.PlatformMessageResponseCallback>(
+      String name, ByteData? data, ui.PlatformMessageResponseCallback callback) {
+    _invoke3<String, ByteData?, ui.PlatformMessageResponseCallback>(
       _onPlatformMessage,
       _onPlatformMessageZone,
       name,
@@ -452,9 +450,9 @@ class EngineWindow extends ui.Window {
 
   @override
   void sendPlatformMessage(
-    String/*!*/ name,
-    ByteData/*?*/ data,
-    ui.PlatformMessageResponseCallback/*?*/ callback,
+    String name,
+    ByteData? data,
+    ui.PlatformMessageResponseCallback? callback,
   ) {
     _sendPlatformMessage(
         name, data, _zonedPlatformMessageResponseCallback(callback));
@@ -462,7 +460,7 @@ class EngineWindow extends ui.Window {
 
   /// Wraps the given [callback] in another callback that ensures that the
   /// original callback is called in the zone it was registered in.
-  static ui.PlatformMessageResponseCallback/*?*/ _zonedPlatformMessageResponseCallback(ui.PlatformMessageResponseCallback/*?*/ callback) {
+  static ui.PlatformMessageResponseCallback? _zonedPlatformMessageResponseCallback(ui.PlatformMessageResponseCallback? callback) {
     if (callback == null) {
       return null;
     }
@@ -470,15 +468,15 @@ class EngineWindow extends ui.Window {
     // Store the zone in which the callback is being registered.
     final Zone registrationZone = Zone.current;
 
-    return (ByteData data) {
+    return (ByteData? data) {
       registrationZone.runUnaryGuarded(callback, data);
     };
   }
 
   void _sendPlatformMessage(
-    String/*!*/ name,
-    ByteData/*?*/ data,
-    ui.PlatformMessageResponseCallback/*?*/ callback,
+    String name,
+    ByteData? data,
+    ui.PlatformMessageResponseCallback? callback,
   ) {
     // In widget tests we want to bypass processing of platform messages.
     if (assertionsEnabled && ui.debugEmulateFlutterTesterEnvironment) {
@@ -497,8 +495,8 @@ class EngineWindow extends ui.Window {
 
     switch (name) {
       case 'flutter/assets':
-        assert(ui.webOnlyAssetManager != null);
-        final String url = utf8.decode(data.buffer.asUint8List());
+        assert(ui.webOnlyAssetManager != null); // ignore: unnecessary_null_comparison
+        final String url = utf8.decode(data!.buffer.asUint8List());
         ui.webOnlyAssetManager.load(url).then((ByteData assetData) {
           _replyToPlatformMessage(callback, assetData);
         }, onError: (dynamic error) {
@@ -519,7 +517,7 @@ class EngineWindow extends ui.Window {
             });
             return;
           case 'HapticFeedback.vibrate':
-            final String type = decoded.arguments;
+            final String? type = decoded.arguments;
             domRenderer.vibrate(_getHapticFeedbackDuration(type));
             _replyToPlatformMessage(
                 callback, codec.encodeSuccessEnvelope(true));
@@ -532,7 +530,7 @@ class EngineWindow extends ui.Window {
                 callback, codec.encodeSuccessEnvelope(true));
             return;
           case 'SystemChrome.setPreferredOrientations':
-            final List<dynamic> arguments = decoded.arguments;
+            final List<dynamic>? arguments = decoded.arguments;
             domRenderer.setPreferredOrientation(arguments).then((bool success) {
               _replyToPlatformMessage(callback,
                 codec.encodeSuccessEnvelope(success));
@@ -559,10 +557,10 @@ class EngineWindow extends ui.Window {
       case 'flutter/mousecursor':
         const MethodCodec codec = StandardMethodCodec();
         final MethodCall decoded = codec.decodeMethodCall(data);
-        final Map<dynamic, dynamic> arguments = decoded.arguments;
+        final Map<dynamic, dynamic>? arguments = decoded.arguments;
         switch (decoded.method) {
           case 'activateSystemCursor':
-            MouseCursor.instance.activateSystemCursor(arguments['kind']);
+            MouseCursor.instance!.activateSystemCursor(arguments!['kind']);
         }
         return;
 
@@ -576,9 +574,9 @@ class EngineWindow extends ui.Window {
 
       case 'flutter/platform_views':
         if (experimentalUseSkia) {
-          rasterizer.viewEmbedder.handlePlatformViewCall(data, callback);
+          rasterizer!.surface.viewEmbedder.handlePlatformViewCall(data, callback);
         } else {
-          ui.handlePlatformViewCall(data, callback);
+          ui.handlePlatformViewCall(data!, callback!);
         }
         return;
 
@@ -592,17 +590,17 @@ class EngineWindow extends ui.Window {
       case 'flutter/navigation':
         const MethodCodec codec = JSONMethodCodec();
         final MethodCall decoded = codec.decodeMethodCall(data);
-        final Map<String, dynamic> message = decoded.arguments;
+        final Map<String, dynamic>? message = decoded.arguments;
         switch (decoded.method) {
           case 'routeUpdated':
           case 'routePushed':
           case 'routeReplaced':
-            _browserHistory.setRouteName(message['routeName']);
+            _browserHistory.setRouteName(message!['routeName']);
             _replyToPlatformMessage(
                 callback, codec.encodeSuccessEnvelope(true));
             break;
           case 'routePopped':
-            _browserHistory.setRouteName(message['previousRouteName']);
+            _browserHistory.setRouteName(message!['previousRouteName']);
             _replyToPlatformMessage(
                 callback, codec.encodeSuccessEnvelope(true));
             break;
@@ -615,7 +613,7 @@ class EngineWindow extends ui.Window {
     }
 
     if (pluginMessageCallHandler != null) {
-      pluginMessageCallHandler(name, data, callback);
+      pluginMessageCallHandler!(name, data, callback);
       return;
     }
 
@@ -629,7 +627,7 @@ class EngineWindow extends ui.Window {
     // callback(null);
   }
 
-  int _getHapticFeedbackDuration(String type) {
+  int _getHapticFeedbackDuration(String? type) {
     switch (type) {
       case 'HapticFeedbackType.lightImpact':
         return DomRenderer.vibrateLightImpact;
@@ -648,8 +646,8 @@ class EngineWindow extends ui.Window {
   /// messages and responses have to be exchanged asynchronously. We simulate
   /// that by adding a zero-length delay to the reply.
   void _replyToPlatformMessage(
-    ui.PlatformMessageResponseCallback callback,
-    ByteData data,
+    ui.PlatformMessageResponseCallback? callback,
+    ByteData? data,
   ) {
     Future<void>.delayed(Duration.zero).then((_) {
       if (callback != null) {
@@ -681,7 +679,7 @@ class EngineWindow extends ui.Window {
   /// A callback that is invoked whenever [_brightnessMediaQuery] changes value.
   ///
   /// Updates the [_platformBrightness] with the new user preference.
-  html.EventListener _brightnessMediaQueryListener;
+  html.EventListener? _brightnessMediaQueryListener;
 
   /// Set the callback function for listening changes in [_brightnessMediaQuery] value.
   void _addBrightnessMediaQueryListener() {
@@ -690,7 +688,7 @@ class EngineWindow extends ui.Window {
         : ui.Brightness.light);
 
     _brightnessMediaQueryListener = (html.Event event) {
-      final html.MediaQueryListEvent mqEvent = event;
+      final html.MediaQueryListEvent mqEvent = event as html.MediaQueryListEvent;
       _updatePlatformBrightness(
           mqEvent.matches ? ui.Brightness.dark : ui.Brightness.light);
     };
@@ -707,34 +705,34 @@ class EngineWindow extends ui.Window {
   }
 
   @override
-  void render(ui.Scene/*!*/ scene) {
+  void render(ui.Scene scene) {
     if (experimentalUseSkia) {
-      final LayerScene layerScene = scene;
-      rasterizer.draw(layerScene.layerTree);
+      final LayerScene layerScene = scene as LayerScene;
+      rasterizer!.draw(layerScene.layerTree);
     } else {
-      final SurfaceScene surfaceScene = scene;
+      final SurfaceScene surfaceScene = scene as SurfaceScene;
       domRenderer.renderScene(surfaceScene.webOnlyRootElement);
     }
   }
 
   @visibleForTesting
-  Rasterizer rasterizer = experimentalUseSkia ? Rasterizer(Surface()) : null;
+  late Rasterizer? rasterizer = experimentalUseSkia ? Rasterizer(Surface(HtmlViewEmbedder())) : null;
 }
 
-bool _handleWebTestEnd2EndMessage(MethodCodec codec, ByteData data) {
+bool _handleWebTestEnd2EndMessage(MethodCodec codec, ByteData? data) {
   final MethodCall decoded = codec.decodeMethodCall(data);
   double ratio = double.parse(decoded.arguments);
   switch (decoded.method) {
     case 'setDevicePixelRatio':
       window.debugOverrideDevicePixelRatio(ratio);
-      window.onMetricsChanged();
+      window.onMetricsChanged!();
       return true;
   }
   return false;
 }
 
 /// Invokes [callback] inside the given [zone].
-void _invoke(void callback(), Zone zone) {
+void _invoke(void callback()?, Zone? zone) {
   if (callback == null) {
     return;
   }
@@ -744,12 +742,12 @@ void _invoke(void callback(), Zone zone) {
   if (identical(zone, Zone.current)) {
     callback();
   } else {
-    zone.runGuarded(callback);
+    zone!.runGuarded(callback);
   }
 }
 
 /// Invokes [callback] inside the given [zone] passing it [arg].
-void _invoke1<A>(void callback(A a), Zone zone, A arg) {
+void _invoke1<A>(void callback(A a)?, Zone? zone, A arg) {
   if (callback == null) {
     return;
   }
@@ -759,13 +757,13 @@ void _invoke1<A>(void callback(A a), Zone zone, A arg) {
   if (identical(zone, Zone.current)) {
     callback(arg);
   } else {
-    zone.runUnaryGuarded<A>(callback, arg);
+    zone!.runUnaryGuarded<A>(callback, arg);
   }
 }
 
 /// Invokes [callback] inside the given [zone] passing it [arg1], [arg2], and [arg3].
 void _invoke3<A1, A2, A3>(
-    void callback(A1 a1, A2 a2, A3 a3), Zone zone, A1 arg1, A2 arg2, A3 arg3) {
+    void callback(A1 a1, A2 a2, A3 a3)?, Zone? zone, A1 arg1, A2 arg2, A3 arg3) {
   if (callback == null) {
     return;
   }
@@ -775,7 +773,7 @@ void _invoke3<A1, A2, A3>(
   if (identical(zone, Zone.current)) {
     callback(arg1, arg2, arg3);
   } else {
-    zone.runGuarded(() {
+    zone!.runGuarded(() {
       callback(arg1, arg2, arg3);
     });
   }
@@ -786,15 +784,15 @@ void _invoke3<A1, A2, A3>(
 /// `dart:ui` window delegates to this value. However, this value has a wider
 /// API surface, providing Web-specific functionality that the standard
 /// `dart:ui` version does not.
-final EngineWindow/*!*/ window = EngineWindow();
+final EngineWindow window = EngineWindow();
 
 /// The Web implementation of [ui.WindowPadding].
 class WindowPadding implements ui.WindowPadding {
   const WindowPadding({
-    this.left,
-    this.top,
-    this.right,
-    this.bottom,
+    required this.left,
+    required this.top,
+    required this.right,
+    required this.bottom,
   });
 
   final double left;
diff --git a/lib/web_ui/lib/ui.dart b/lib/web_ui/lib/ui.dart
index bfedc844f9ae4..49217e14a861d 100644
--- a/lib/web_ui/lib/ui.dart
+++ b/lib/web_ui/lib/ui.dart
@@ -48,7 +48,7 @@ void webOnlyInitializeEngine() {
   engine.initializeEngine();
 }
 
-void webOnlySetPluginHandler(Future<void> Function(String, ByteData, PlatformMessageResponseCallback) handler) {
+void webOnlySetPluginHandler(Future<void> Function(String, ByteData?, PlatformMessageResponseCallback?) handler) {
   engine.pluginMessageCallHandler = handler;
 }
 
diff --git a/lib/web_ui/test/canvaskit/path_metrics_test.dart b/lib/web_ui/test/canvaskit/path_metrics_test.dart
index 9089a6b57af24..b2c3529f8f082 100644
--- a/lib/web_ui/test/canvaskit/path_metrics_test.dart
+++ b/lib/web_ui/test/canvaskit/path_metrics_test.dart
@@ -62,8 +62,8 @@ void main() {
       expect(iter2.current, isNotNull);
       expect(iter1.moveNext(), false);
       expect(iter2.moveNext(), false);
-      expect(iter1.current, isNull);
-      expect(iter2.current, isNull);
+      expect(() => iter1.current, throwsRangeError);
+      expect(() => iter2.current, throwsRangeError);
     });
   },
       // This test failed on iOS Safari.
diff --git a/lib/web_ui/test/dom_renderer_test.dart b/lib/web_ui/test/dom_renderer_test.dart
index 1c1d53c5c4a21..ca9d6804a47c8 100644
--- a/lib/web_ui/test/dom_renderer_test.dart
+++ b/lib/web_ui/test/dom_renderer_test.dart
@@ -76,29 +76,6 @@ void main() {
     renderer.detachElement(child);
     expect(element.children, isEmpty);
   });
-  test('can reattach detached elements', () {
-    final DomRenderer renderer = DomRenderer();
-    final html.Element element = renderer.createElement('div');
-    final html.Element child = renderer.createElement('div', parent: element);
-    final html.Element otherChild =
-        renderer.createElement('foo', parent: element);
-    renderer.detachElement(child);
-    expect(element.children, hasLength(1));
-    renderer.attachBeforeElement(element, otherChild, child);
-    expect(element.children, hasLength(2));
-  });
-  test('insert two elements in the middle of a child list', () {
-    final DomRenderer renderer = DomRenderer();
-    final html.Element parent = renderer.createElement('div');
-    renderer.createElement('a', parent: parent);
-    final html.Element childD = renderer.createElement('d', parent: parent);
-    expect(parent.innerHtml, '<a></a><d></d>');
-    final html.Element childB = renderer.createElement('b', parent: parent);
-    final html.Element childC = renderer.createElement('c', parent: parent);
-    renderer.attachBeforeElement(parent, childD, childB);
-    renderer.attachBeforeElement(parent, childD, childC);
-    expect(parent.innerHtml, '<a></a><b></b><c></c><d></d>');
-  });
 
   test('innerHeight/innerWidth are equal to visualViewport height and width',
       () {
diff --git a/lib/web_ui/test/paragraph_test.dart b/lib/web_ui/test/paragraph_test.dart
index cc3f16663b4d2..0bb402651fae4 100644
--- a/lib/web_ui/test/paragraph_test.dart
+++ b/lib/web_ui/test/paragraph_test.dart
@@ -406,9 +406,9 @@ void main() async {
     expect(
       paragraph.getBoxesForRange(1, 2).single,
       const TextBox.fromLTRBD(
-        10,
+        970,
         0,
-        20,
+        980,
         10,
         TextDirection.rtl,
       ),