Enable setting the UINode key for ParentDataNodes

sky/sdk/lib/framework/components2/menu_item.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 'button_base.dart';
 import 'icon.dart';
 import 'ink_well.dart';
 
 class MenuItem extends ButtonBase {
 
   static const BoxDecoration highlightDecoration = const BoxDecoration(
     backgroundColor: const Color.fromARGB(102, 153, 153, 153)
-  ); 
+  );
 
   List<UINode> children;
   String icon;
   GestureEventListener onGestureTap;
 
   MenuItem({ Object key, this.icon, this.children, this.onGestureTap }) : super(key: key);
 
   UINode buildContent() {
     return new EventListenerNode(
       new Container(
         child: new InkWell(
           children: [
             new Padding(
               child: new Icon(type: "${icon}_grey600", size: 24),
               padding: const EdgeDims.symmetric(horizontal: 16.0)
             ),
             new FlexExpandingChild(
               new Padding(
                 child: new FlexContainer(
                   direction: FlexDirection.horizontal,
                   children: children
                 ),
                 padding: const EdgeDims.symmetric(horizontal: 16.0)
               ),
-              1
+              flex: 1
             )
           ]
         ),
         height: 48.0,
         decoration: highlight ? highlightDecoration : null
       ),
       onGestureTap: onGestureTap
     );
   }
 }