Implement Scene.toImage() in CanvasKit mode.

lib/web_ui/lib/src/engine/canvaskit/layer_scene_builder.dart

@@ -17,7 +17,8 @@ class LayerScene implements ui.Scene {
 
   @override
   Future<ui.Image> toImage(int width, int height) {
-    throw UnsupportedError('LayerScene.toImage not implemented.');
+    ui.Picture picture = layerTree.flatten();
+    return picture.toImage(width, height);
   }
 }
 

lib/web_ui/lib/src/engine/canvaskit/layer_tree.dart

@@ -52,6 +52,27 @@ class LayerTree {
       rootLayer!.paint(context);
     }
   }
+
+  /// Flattens the tree into a single [ui.Picture].
+  ///
+  /// This picture does not contain any platform views.
+  ui.Picture flatten() {
+    CkPictureRecorder recorder = CkPictureRecorder();
+    CkCanvas canvas = recorder.beginRecording(ui.Rect.largest);
+    if (rootLayer != null) {
+      final PrerollContext prerollContext = PrerollContext(null, null);
+      rootLayer!.preroll(prerollContext, Matrix4.identity());
+
+      CkNWayCanvas internalNodesCanvas = CkNWayCanvas();
+      internalNodesCanvas.addCanvas(canvas);
+      final PaintContext paintContext =
+          PaintContext(internalNodesCanvas, canvas, null, null);
+      if (rootLayer!.needsPainting) {
+        rootLayer!.paint(paintContext);
+      }
+    }
+    return recorder.endRecording();
+  }
 }
 
 /// A single frame to be rendered.

lib/web_ui/test/canvaskit/scene_test.dart

@@ -0,0 +1,53 @@
+// 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
+import 'package:test/bootstrap/browser.dart';
+import 'package:test/test.dart';
+
+import 'package:ui/src/engine.dart';
+import 'package:ui/ui.dart' as ui;
+
+import 'common.dart';
+
+void main() {
+  internalBootstrapBrowserTest(() => testMain);
+}
+
+void testMain() {
+  group('LayerScene', () {
+    setUpAll(() async {
+      await ui.webOnlyInitializePlatform();
+    });
+
+    test('toImage returns an image', () async {
+      final ui.PictureRecorder recorder = ui.PictureRecorder();
+      expect(recorder, isA<CkPictureRecorder>());
+
+      final ui.Canvas canvas = ui.Canvas(recorder);
+      expect(canvas, isA<CanvasKitCanvas>());
+
+      final ui.Paint paint = ui.Paint();
+      expect(paint, isA<CkPaint>());
+      paint.color = ui.Color.fromARGB(255, 255, 0, 0);
+
+      // Draw a red circle.
+      canvas.drawCircle(ui.Offset(20, 20), 10, paint);
+
+      final ui.Picture picture = recorder.endRecording();
+      expect(picture, isA<CkPicture>());
+
+      final ui.SceneBuilder builder = ui.SceneBuilder();
+      expect(builder, isA<LayerSceneBuilder>());
+
+      builder.pushOffset(0, 0);
+      builder.addPicture(ui.Offset(0, 0), picture);
+
+      final ui.Scene scene = builder.build();
+
+      final ui.Image sceneImage = await scene.toImage(100, 100);
+      expect(sceneImage, isA<CkImage>());
+    });
+  });
+}