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

 // 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:async';
 import 'dart:collection';
 import 'dart:mirrors';
 import 'dart:sky' as sky;
 
 import '../app.dart';
@@ skipping 10 matching lines @@
 final bool _shouldLogRenderDuration = false;
 
 /*
  * All Effen nodes derive from UINode. All nodes have a _parent, a _key and
  * can be sync'd.
  */
 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;
   String get key => _key;
 
   UINode _parent;
   UINode get parent => _parent;
 
   bool _mounted = false;
   bool _wasMounted = false;
@@ skipping 703 matching lines @@
 }
 
 
 class UINodeAppView extends AppView {
 
   UINodeAppView() {
     assert(_appView == null);
   }
 
   static UINodeAppView _appView;
+  static AppView get appView => _appView;
   static void initUINodeAppView() {
     if (_appView == null)
       _appView = new UINodeAppView();
   }
 
   void dispatchEvent(sky.Event event, HitTestResult result) {
     assert(_appView == this);
     super.dispatchEvent(event, result);
     for (HitTestEntry entry in result.path.reversed) {
       UINode target = RenderObjectWrapper._getMounted(entry.target);
@@ skipping 29 matching lines @@
     assert(parent == null);
     _sync(null, null);
   }
 
 }
 
 abstract class App extends AbstractUINodeRoot {
 
   App();
 
-  AppView get appView => UINodeAppView._appView;
-
   void _buildIfDirty() {
     super._buildIfDirty();
 
     if (root.parent == null) {
       // we haven't attached it yet
       UINodeAppView._appView.root = root;
     }
     assert(root.parent is RenderView);
   }
 
@@ skipping 35 matching lines @@
       // we haven't attached it yet
       assert(_container.child == null);
       _container.child = root;
     }
     assert(root.parent == _container);
   }
 
   UINode build() => builder();
 
 }