Make UINode's key a String

sky/sdk/lib/widgets/modal_overlay.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 'ui_node.dart';
 
 class ModalOverlay extends Component {
 
-  ModalOverlay({ Object key, this.children, this.onDismiss }) : super(key: key);
+  ModalOverlay({ String key, this.children, this.onDismiss }) : super(key: key);
 
   // static final Style _style = new Style('''
   //   position: absolute;
   //   top: 0;
   //   left: 0;
   //   bottom: 0;
   //   right: 0;''');
 
   final List<UINode> children;
   final GestureEventListener onDismiss;
 
   UINode build() {
     return new EventListenerNode(
       new Stack(children),
       onGestureTap: onDismiss);
   }
 
 }