Fix the crash when the drawer is hidden.

sky/sdk/lib/framework/components2/scaffold.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 '../fn2.dart';
 import 'dart:sky' as sky;
 import '../rendering/box.dart';
 import '../rendering/node.dart';
 
 
@@ skipping 30 matching lines @@
     if (old != null)
       dropChild(old);
     _slots[slot] = value;
     if (value != null)
       adoptChild(value);
     markNeedsLayout();
   }
 
   ScaffoldSlots remove(RenderBox child) {
     assert(child != null);
-    for (ScaffoldSlots slot in ScaffoldSlots) {
+    for (ScaffoldSlots slot in ScaffoldSlots.values) {
       if (_slots[slot] == child) {
         this[slot] = null;
         return slot;
       }
     }
     return null;
   }
 
   bool get sizedByParent => true;
   void performResize() {
@@ skipping 109 matching lines @@
   void syncRenderNode(UINode old) {
     super.syncRenderNode(old);
     syncChild(toolbar, old is Scaffold ? old.toolbar : null, ScaffoldSlots.Toolbar);
     syncChild(body, old is Scaffold ? old.body : null, ScaffoldSlots.Body);
     syncChild(statusbar, old is Scaffold ? old.statusbar : null, ScaffoldSlots.StatusBar);
     syncChild(drawer, old is Scaffold ? old.drawer : null, ScaffoldSlots.Drawer);
     syncChild(floatingActionButton, old is Scaffold ? old.floatingActionButton : null, ScaffoldSlots.FloatingActionButton);
   }
 
 }