Introduce RenderNodeWithChildMixin

sky/sdk/lib/framework/app.dart

Index: sky/sdk/lib/framework/app.dart
diff --git a/sky/sdk/lib/framework/app.dart b/sky/sdk/lib/framework/app.dart
index 9c3815a999f71b94412609682d313ee402f9e877..4b245089421f632b82cdad0504aae9dd94ad50ec 100644
--- a/sky/sdk/lib/framework/app.dart
+++ b/sky/sdk/lib/framework/app.dart
@@ -11,7 +11,7 @@ class AppView {
     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);
   
@@ -20,9 +20,9 @@ class AppView {
 
   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) {