Make UINode's key a String

sky/sdk/lib/widgets/button_base.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';
 
 abstract class ButtonBase extends Component {
 
-  ButtonBase({ Object key, this.highlight: false }) : super(key: key);
+  ButtonBase({ String key, this.highlight: false }) : super(key: key);
 
   bool highlight;
 
   void syncFields(ButtonBase source) {
     highlight = source.highlight;
   }
 
   void _handlePointerDown(_) {
     setState(() {
       highlight = true;
@@ skipping 15 matching lines @@
       buildContent(),
       onPointerDown: _handlePointerDown,
       onPointerUp: _handlePointerUp,
       onPointerCancel: _handlePointerCancel
     );
   }
 
   UINode buildContent();
 
 }