Make UINode's key a String

sky/sdk/lib/widgets/scrollable.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:math' as math;
 import 'dart:sky' as sky;
 
 import '../framework/animation/generators.dart';
 import '../framework/animation/mechanics.dart';
 import '../framework/animation/scroll_behavior.dart';
 import '../theme2/edges.dart';
 import '../theme2/view_configuration.dart' as config;
 import 'basic.dart';
 import 'material.dart';
 
 const double _kMillisecondsPerSecond = 1000.0;
 
 double _velocityForFlingGesture(sky.GestureEvent event) {
   return math.max(-config.kMaxFlingVelocity, math.min(config.kMaxFlingVelocity,
       -event.velocityY)) / _kMillisecondsPerSecond;
 }
 
 abstract class ScrollClient {
   bool ancestorScrolled(Scrollable ancestor);
 }
 
 abstract class Scrollable extends Component {
 
-  Scrollable({ Object key, Color this.backgroundColor }) : super(key: key, stateful: true);
+  Scrollable({ String key, Color this.backgroundColor })
+    : super(key: key, stateful: true);
 
   Color backgroundColor;
 
   void syncFields(Scrollable source) {
     backgroundColor = source.backgroundColor;
   }
 
   double _scrollOffset = 0.0;
   double get scrollOffset => _scrollOffset;
 
@@ skipping 114 matching lines @@
 
   void _handleFlingCancel(sky.GestureEvent event) {
     _startSimulation(_createParticle());
   }
 
   void _handleWheel(sky.WheelEvent event) {
     scrollBy(-event.offsetY);
   }
 
 }