[Effen] fix _ensureDepth to work on root of tree, oops

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 76 matching lines @@
   void _sync(UINode old, sky.ParentNode host, sky.Node insertBefore);
 
   void _remove() {
     _defunct = true;
     _root = null;
   }
 
   int _nodeDepth;
   void _ensureDepth() {
     if (_nodeDepth == null) {
-      _parent.ensureDepth();
-      _nodeDepth = _parent._nodeDepth + 1;
+      if (_parent != null) {
+        _parent.ensureDepth();
+        _nodeDepth = _parent._nodeDepth + 1;
+      } else {
+        _nodeDepth = 0;
+      }
     }
   }
 
   void _trace(String message) {
     if (!_shouldTrace)
       return;
 
     _ensureDepth();
     print((' ' * _nodeDepth) + message);
   }
@@ skipping 780 matching lines @@
 
 abstract class App extends Component {
   sky.Node _host;
 
   App() : super(stateful: true) {
     _host = sky.document.createElement('div');
     sky.document.appendChild(_host);
     _scheduleComponentForRender(this);
   }
 }