Make UINode's key a String

sky/sdk/lib/widgets/drawer_header.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 '../theme2/colors.dart';
 import '../theme2/view_configuration.dart';
 import 'basic.dart';
 
 class DrawerHeader extends Component {
 
-  DrawerHeader({ Object key, this.children }) : super(key: key);
+  DrawerHeader({ String key, this.children }) : super(key: key);
 
   final List<UINode> children;
 
   UINode build() {
     return new Container(
       height: kStatusBarHeight + kMaterialDrawerHeight,
       decoration: new BoxDecoration(
         backgroundColor: BlueGrey[50],
         border: const Border(
           bottom: const BorderSide(
@@ skipping 10 matching lines @@
           key: 'drawer-header-label',
           padding: const EdgeDims.symmetric(horizontal: 16.0),
           child: new Flex(children, direction: FlexDirection.horizontal)
         )],
         direction: FlexDirection.vertical
       )
     );
   }
 
 }