Make UINode's key a String

sky/sdk/lib/widgets/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 'basic.dart';
 import 'button_base.dart';
 import 'icon.dart';
 import 'ink_well.dart';
 import 'ui_node.dart';
 
 const BoxDecoration _kHighlightDecoration = const BoxDecoration(
   backgroundColor: const Color.fromARGB(102, 153, 153, 153)
 );
 
 // TODO(abarth): We shouldn't need _kHighlightBoring, but currently Container
 //               isn't smart enough to retain the components it builds when we
 //               add or remove a |decoration|. For now, we use a transparent
 //               decoration to avoid changing the structure of the tree. The
 //               right fix, however, is to make Container smarter about how it
 //               syncs its subcomponents.
 const BoxDecoration _kHighlightBoring = const BoxDecoration(
   backgroundColor: const Color.fromARGB(0, 0, 0, 0)
 );
 
 class MenuItem extends ButtonBase {
-  MenuItem({ Object key, this.icon, this.children, this.onGestureTap }) : super(key: key);
+  MenuItem({ String key, this.icon, this.children, this.onGestureTap })
+    : super(key: key);
 
   String icon;
   List<UINode> children;
   GestureEventListener onGestureTap;
 
   void syncFields(MenuItem source) {
     icon = source.icon;
     children = source.children;
     onGestureTap = source.onGestureTap;
     super.syncFields(source);
@@ skipping 17 matching lines @@
             )
           ])
         ),
         height: 48.0,
         decoration: highlight ? _kHighlightDecoration : _kHighlightBoring
       ),
       onGestureTap: onGestureTap
     );
   }
 }