Introduce RenderProxyBox and RenderSizedBox

sky/examples/raw/simple_render_tree.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';
 import 'package:sky/framework/app.dart';
 import 'package:sky/framework/layout2.dart';
 
 class RenderSolidColor extends RenderDecoratedBox {
   final Size desiredSize;
   final int backgroundColor;
 
-  RenderSolidColor(int backgroundColor, { this.desiredSize })
+  RenderSolidColor(int backgroundColor, { this.desiredSize: const sky.Size.infinite() })
       : backgroundColor = backgroundColor,
         super(new BoxDecoration(backgroundColor: backgroundColor)) {
   }
 
   BoxDimensions getIntrinsicDimensions(BoxConstraints constraints) {
     return new BoxDimensions.withConstraints(constraints,
                                              width: desiredSize.width,
                                              height: desiredSize.height);
   }
 
@@ skipping 10 matching lines @@
 }
 
 AppView app;
 
 void main() {
   var root = new RenderFlex(
       direction: FlexDirection.Vertical,
       decoration: new BoxDecoration(backgroundColor: 0xFF000000));
 
   void addFlexChild(RenderFlex parent, int backgroundColor, { int flex: 0 }) {
-    RenderNode child = new RenderSolidColor(backgroundColor, desiredSize: new Size.infinite());
+    RenderNode child = new RenderSolidColor(backgroundColor);
     parent.add(child);
     child.parentData.flex = flex;
   }
 
   // Yellow bar at top
   addFlexChild(root, 0xFFFFFF00, flex: 1);
 
   // Turquoise box
   root.add(new RenderSolidColor(0x7700FFFF, desiredSize: new Size(100.0, 100.0)));
 
@@ skipping 12 matching lines @@
   // Purple and blue cells
   addFlexChild(row, 0x77FF00FF, flex: 1);
   addFlexChild(row, 0xFF0000FF, flex: 2);
 
   root.add(row);
   row.parentData.flex = 3;
 
   app = new AppView(root);
 
 }