Make UINode's key a String

sky/sdk/lib/widgets/raised_button.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/edges.dart';
 import 'basic.dart';
 import 'button_base.dart';
 import 'ink_well.dart';
 import 'material.dart';
 
 enum RaisedButtonTheme { light, dark }
 
 class RaisedButton extends ButtonBase {
 
   RaisedButton({
-    Object key,
+    String key,
     this.child,
     this.enabled: true,
     this.onPressed,
     this.theme: RaisedButtonTheme.light
   }) : super(key: key);
 
   UINode child;
   bool enabled;
   Function onPressed;
   RaisedButtonTheme theme;
@@ skipping 40 matching lines @@
           child: enabled ? new InkWell(child: contents) : contents,
           level: enabled ? (highlight ? 2 : 1) : 0,
           color: color
         )
       ),
       onGestureTap: (_) { if (onPressed != null && enabled) onPressed(); }
     );
   }
 
 }