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.