Call _notifyMountStatusChanged() on App startup

sky/framework/fn.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.
 
 library fn;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:sky' as sky;
 import 'reflect.dart' as reflect;
@@ skipping 749 matching lines @@
   sky.Node _host = null;
   App() : super(stateful: true) {
     _host = sky.document.createElement('div');
     sky.document.appendChild(_host);
 
     new Future.microtask(() {
       Stopwatch sw = new Stopwatch()..start();
 
       _sync(null, _host, null);
       assert(_root is sky.Node);
+      _notifyMountStatusChanged();
 
       sw.stop();
       if (_shouldLogRenderDuration)
         print("Initial build: ${sw.elapsedMicroseconds} microseconds");
     });
   }
 }