Introduce RenderProxyBox and RenderSizedBox

sky/examples/raw/spinning_square.dart

Index: sky/examples/raw/spinning_square.dart
diff --git a/sky/examples/raw/spinning_square.dart b/sky/examples/raw/spinning_square.dart
index 155f5ccc685d798a0cd0d786fb2f867685e4eb85..17072330d56a229ea035b708aba5d86df568c8ef 100644
--- a/sky/examples/raw/spinning_square.dart
+++ b/sky/examples/raw/spinning_square.dart
@@ -13,7 +13,7 @@ void beginFrame(double timeStamp) {
   PictureRecorder canvas = new PictureRecorder(view.width, view.height);
   canvas.translate(view.width / 2.0, view.height / 2.0);
   canvas.rotateDegrees(delta / 10);
-  canvas.drawRect(new Rect()..setLTRB(-100.0, -100.0, 100.0, 100.0),
+  canvas.drawRect(new Rect.fromLTRB(-100.0, -100.0, 100.0, 100.0),
                   new Paint()..setARGB(255, 0, 255, 0));
   view.picture = canvas.endRecording();
   view.scheduleFrame();