Abstract out the AppView logic in fn so that it can also be used in non-pure-fn apps.

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 'dart:sky' as sky;
 import 'rendering/box.dart';
 import 'rendering/object.dart';
 import 'scheduler.dart' as scheduler;
 
 class PointerState {
   HitTestResult result;
   Point lastPosition;
 
   PointerState({ this.result, this.lastPosition });
 }
 
 class AppView {
 
-  AppView(RenderBox root) {
+  AppView([RenderBox root = null]) {
+    assert(_app == null);
+    _app = this;
+
     sky.view.setEventCallback(_handleEvent);
     sky.view.setMetricsChangedCallback(_handleMetricsChanged);
     scheduler.init();
     scheduler.addPersistentFrameCallback(_beginFrame);
 
     _renderView = new RenderView(child: root);
     _renderView.attach();
     _renderView.rootConstraints = _viewConstraints;
     _renderView.scheduleInitialLayout();
+
+    assert(_app == this);
   }
 
+  static AppView _app; // used to enforce that we're a singleton
+
   RenderView _renderView;
 
   ViewConstraints get _viewConstraints =>
       new ViewConstraints(width: sky.view.width, height: sky.view.height);
 
   Map<int, PointerState> _stateForPointer = new Map<int, PointerState>();
 
   Function onFrame;
 
   RenderBox get root => _renderView.child;
@@ skipping 62 matching lines @@
       entry.target.handleEvent(event, entry);
   }
 
   String toString() => 'Render Tree:\n${_renderView}';
 
   void debugDumpRenderTree() {
     toString().split('\n').forEach(print);
   }
   
 }