Make UINode's key a String

sky/sdk/lib/widgets/toggleable.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 'dart:sky' as sky;
 
 import 'animated_component.dart';
 import 'basic.dart';
 import '../framework/animation/animated_value.dart';
 import '../framework/animation/curves.dart';
 
 typedef void ValueChanged(value);
 
 const double _kCheckDuration = 200.0;
 
 abstract class Toggleable extends AnimatedComponent {
 
   Toggleable({
-    Object key,
+    String key,
     this.value,
     this.onChanged
   }) : super(key: key) {
     toggleAnimation = new AnimatedValue(value ? 1.0 : 0.0);
   }
 
   bool value;
   AnimatedValue toggleAnimation;
   ValueChanged onChanged;
 
@@ skipping 35 matching lines @@
         height: size.height,
         child: new CustomPaint(
           token: toggleAnimation.value,
           callback: customPaintCallback
         )
       ),
       onGestureTap: _handleClick
     );
   }
 }