Make UINode's key a String

sky/sdk/lib/widgets/animated_component.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:async';
 
 import '../framework/animation/animated_value.dart';
 import 'basic.dart';
 
 typedef void SetterFunction(double value);
 
 class _AnimationEntry {
   _AnimationEntry(this.value, this.setter);
   final AnimatedValue value;
   final SetterFunction setter;
   StreamSubscription<double> subscription;
 }
 
 abstract class AnimatedComponent extends Component {
 
-  AnimatedComponent({ Object key }) : super(key: key, stateful: true);
+  AnimatedComponent({ String key }) : super(key: key, stateful: true);
 
   void syncFields(AnimatedComponent source) { }
 
   List<_AnimationEntry> _animatedFields = new List<_AnimationEntry>();
 
   animate(AnimatedValue value, SetterFunction setter) {
     assert(!mounted);
     setter(value.value);
     _animatedFields.add(new _AnimationEntry(value, setter));
   }
@@ skipping 11 matching lines @@
   void didUnmount() {
     for (_AnimationEntry entry in _animatedFields) {
       assert(entry.subscription != null);
       entry.subscription.cancel();
       entry.subscription = null;
     }
     super.didUnmount();
   }
 
 }