Make UINode's key a String

sky/sdk/lib/widgets/menu_divider.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';
 
 class MenuDivider extends Component {
-  MenuDivider({ Object key }) : super(key: key);
+  MenuDivider({ String key }) : super(key: key);
 
   UINode build() {
     return new Container(
       height: 0.0,
       decoration: const BoxDecoration(
         border: const Border(
           bottom: const BorderSide(
             color: const Color.fromARGB(31, 0, 0, 0)
           )
         )
       ),
       margin: const EdgeDims.symmetric(vertical: 8.0)
     );
   }
 }