Introduce RenderNodeWithChildMixin

sky/sdk/lib/framework/app.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 'layout2.dart';
 import 'dart:sky' as sky;
 
 class AppView {
 
   AppView(RenderBox root) {
     sky.view.setEventCallback(_handleEvent);
     sky.view.setBeginFrameCallback(_beginFrame);
 
-    _renderView = new RenderView(root: root);
+    _renderView = new RenderView(child: root);
     _renderView.attach();
     _renderView.layout(newWidth: sky.view.width, newHeight: sky.view.height);
   
     sky.view.scheduleFrame();
   }
 
   RenderView _renderView;
 
-  RenderBox get root => _renderView.root;
+  RenderBox get root => _renderView.child;
   void set root(RenderBox value) {
-    _renderView.root = value;
+    _renderView.child = value;
   }
 
   void _beginFrame(double timeStamp) {
     RenderNode.flushLayout();
     _renderView.paintFrame();
   }
 
   void _handleEvent(sky.Event event) {
     if (event is! sky.PointerEvent)
       return;
     HitTestResult result = new HitTestResult();
     _renderView.hitTest(result, x: event.x, y: event.y);
     result.path.reversed.forEach((RenderNode node) {
       node.handlePointer(event);
     });
   }
 
 }