Expose the RenderView root through the AppView, so that you can change it.

Also, attach the RenderView so that it actually updates during a flush layout.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1149063007

sdk/lib/framework/app.dart

@@ -12,6 +12,7 @@ class AppView {
     sky.view.setBeginFrameCallback(_beginFrame);
 
     _renderView = new RenderView(root: root);
+    _renderView.attach();
     _renderView.layout(newWidth: sky.view.width, newHeight: sky.view.height);
   
     sky.view.scheduleFrame();
@@ -19,6 +20,11 @@ class AppView {
 
   RenderView _renderView;
 
+  RenderBox get root => _renderView.root;
+  void set root(RenderBox value) {
+    _renderView.root = value;
+  }
+
   void _beginFrame(double timeStamp) {
     RenderNode.flushLayout();
     _renderView.paintFrame();