Make it easier to debug examples/widgets/container.dart, since it is currently showing a crazy clip…

sky/examples/widgets/spinning_mixed.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:sky' as sky;
 
 import 'package:sky/framework/rendering/box.dart';
 import 'package:sky/framework/rendering/flex.dart';
 import 'package:sky/framework/scheduler.dart';
 import 'package:sky/framework/widgets/raised_button.dart';
 import 'package:sky/framework/widgets/ui_node.dart';
 import 'package:sky/framework/widgets/wrappers.dart';
 import 'package:vector_math/vector_math.dart';
 
 import '../lib/solid_color_box.dart';
+import '../../tests/resources/display_list.dart';
 
 // Solid colour, RenderObject version
 void addFlexChildSolidColor(RenderFlex parent, sky.Color backgroundColor, { int flex: 0 }) {
   RenderSolidColorBox child = new RenderSolidColorBox(backgroundColor);
   parent.add(child);
   child.parentData.flex = flex;
 }
 
 // Solid colour, Widget version
 class Rectangle extends Component {
@@ skipping 26 matching lines @@
       new Rectangle(const Color(0xFFFFFF00), key: 'b'),
     ],
     direction: FlexDirection.vertical,
     justifyContent: FlexJustifyContent.spaceBetween
   );
 }
 
 double timeBase;
 RenderTransform transformBox;
 
+final TestRenderView tester = new TestRenderView();
+
 void rotate(double timeStamp) {
   if (timeBase == null)
     timeBase = timeStamp;
   double delta = (timeStamp - timeBase) / 1000; // radians
 
   transformBox.setIdentity();
   transformBox.translate(transformBox.size.width / 2.0, transformBox.size.height / 2.0);
   transformBox.rotateZ(delta);
   transformBox.translate(-transformBox.size.width / 2.0, -transformBox.size.height / 2.0);
+
+  // tester.checkFrame();
 }
 
+final bool debugDisplayList = false; // set this to true to use the test rendering logic
+
 void main() {
+  // Because we're going to use UINodes, we want to initialise its
+  // AppView, not use the default one. We don't really need to do
+  // this, because RenderObjectToUINodeAdapter does it for us, but
+  // it's good practice in case we happen to not have a
+  // RenderObjectToUINodeAdapter in our tree at startup, or in case we
+  // want a renderViewOverride.
+  UINodeAppView.initUINodeAppView(renderViewOverride: debugDisplayList ? tester : null);
+
   RenderFlex flexRoot = new RenderFlex(direction: FlexDirection.vertical);
 
   RenderProxyBox proxy = new RenderProxyBox();
   new RenderObjectToUINodeAdapter(proxy, builder); // adds itself to proxy
 
   addFlexChildSolidColor(flexRoot, const sky.Color(0xFFFF00FF), flex: 1);
   flexRoot.add(proxy);
   addFlexChildSolidColor(flexRoot, const sky.Color(0xFF0000FF), flex: 1);
 
   transformBox = new RenderTransform(child: flexRoot, transform: new Matrix4.identity());
   RenderPadding root = new RenderPadding(padding: new EdgeDims.all(20.0), child: transformBox);
 
-  // Because we're going to use UINodes, we want to initialise its
-  // AppView, not use the default one. We don't really need to do
-  // this, because RenderObjectToUINodeAdapter does it for us, but
-  // it's good practice in case we happen to not have a
-  // RenderObjectToUINodeAdapter in our tree at startup.
-  UINodeAppView.initUINodeAppView();
   UINodeAppView.appView.root = root;
-
   addPersistentFrameCallback(rotate);
 }