Add an example of an app that manipulates both a RenderObject tree and has some fn logic in it.

sky/sdk/lib/framework/widgets/ui_node.dart

Index: sky/sdk/lib/framework/widgets/ui_node.dart
diff --git a/sky/sdk/lib/framework/widgets/ui_node.dart b/sky/sdk/lib/framework/widgets/ui_node.dart
index 2d1cd2497b73444203874a6e78180409b87536df..d8d5968fe3ac84b0944c0f51cf70afe5cdbf3b21 100644
--- a/sky/sdk/lib/framework/widgets/ui_node.dart
+++ b/sky/sdk/lib/framework/widgets/ui_node.dart
@@ -28,7 +28,7 @@ abstract class UINode {
 
   UINode({ Object key }) {
     _key = key == null ? "$runtimeType" : "$runtimeType-$key";
-    assert(this is App || _inRenderDirtyComponents); // you should not build the UI tree ahead of time, build it only during build()
+    assert(this is AbstractUINodeRoot || _inRenderDirtyComponents); // you should not build the UI tree ahead of time, build it only during build()
   }
 
   String _key;
@@ -752,6 +752,7 @@ class UINodeAppView extends AppView {
   }
 
   static UINodeAppView _appView;
+  static AppView get appView => _appView;
   static void initUINodeAppView() {
     if (_appView == null)
       _appView = new UINodeAppView();
@@ -801,8 +802,6 @@ abstract class App extends AbstractUINodeRoot {
 
   App();
 
-  AppView get appView => UINodeAppView._appView;
-
   void _buildIfDirty() {
     super._buildIfDirty();