fix "TREE INCONSISTENT" noise in compositing_test.dart

lib/web_ui/lib/src/engine/surface/picture.dart

@@ -356,6 +356,11 @@ abstract class PersistedPicture extends PersistedLeafSurface {
 
   EngineCanvas _canvas;
 
+  /// Returns the canvas used by this picture layer.
+  ///
+  /// Useful for tests.
+  EngineCanvas get debugCanvas => _canvas;
+
   final double dx;
   final double dy;
   final EnginePicture picture;
@@ -624,7 +629,7 @@ abstract class PersistedPicture extends PersistedLeafSurface {
     super.debugValidate(validationErrors);
 
     if (picture.recordingCanvas.didDraw) {
-      if (_canvas == null) {
+      if (debugCanvas == null) {
         validationErrors
             .add('$runtimeType has non-trivial picture but it has null canvas');
       }

lib/web_ui/test/compositing_test.dart

@@ -151,7 +151,7 @@ void main() {
 
       scene2.preroll();
       scene2.update(scene1);
-      commitScene(scene1);
+      commitScene(scene2);
       expect(picture.retainCount, 1);
       expect(picture.buildCount, 1);
       expect(picture.updateCount, 0);
@@ -169,7 +169,7 @@ void main() {
 
       scene3.preroll();
       scene3.update(scene2);
-      commitScene(scene1);
+      commitScene(scene3);
       expect(picture.retainCount, 2);
       expect(picture.buildCount, 1);
       expect(picture.updateCount, 0);
@@ -271,6 +271,13 @@ class MockPersistedPicture extends PersistedPicture {
   int updateCount = 0;
   int applyPaintCount = 0;
 
+  final BitmapCanvas _fakeCanvas = BitmapCanvas(const Rect.fromLTRB(0, 0, 10, 10));
+
+  @override
+  EngineCanvas get debugCanvas {
+    return _fakeCanvas;
+  }
+
   @override
   double matchForUpdate(PersistedPicture existingSurface) {
     return identical(existingSurface.picture, picture) ? 0.0 : 1.0;