Make UINode's key a String

sky/examples/stocks2/lib/stock_menu.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 'package:sky/widgets/checkbox.dart';
 import 'package:sky/widgets/popup_menu.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/framework/theme/view_configuration.dart';
 
 class StockMenu extends Component {
 
   StockMenu({
-    Object key,
+    String key,
     this.controller,
     this.autorefresh: false,
     this.onAutorefreshChanged
   }) : super(key: key);
 
   final PopupMenuController controller;
   final bool autorefresh;
   final ValueChanged onAutorefreshChanged;
 
   UINode build() {
@@ skipping 10 matching lines @@
           [new Text('Remove stock')],
           [new Flexible(child: new Text('Autorefresh')), checkbox],
         ],
         level: 4
       ),
       right: 8.0,
       top: 8.0 + kStatusBarHeight
     );
   }
 }