Make UINode's key a String

sky/sdk/lib/widgets/drawer.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 'package:vector_math/vector_math.dart';
 
 import '../framework/animation/animated_value.dart';
@@ skipping 91 matching lines @@
 
     if (distance > 0)
       position.animateTo(targetPosition, duration, curve: linear);
   }
 
 }
 
 class Drawer extends AnimatedComponent {
 
   Drawer({
-    Object key,
+    String key,
     this.controller,
     this.children,
     this.level: 0
   }) : super(key: key) {
     animate(controller.position, (double value) {
       _position = value;
     });
   }
 
   List<UINode> children;
@@ skipping 34 matching lines @@
     return new EventListenerNode(
       new Stack([ mask, content ]),
       onPointerDown: controller.handlePointerDown,
       onPointerMove: controller.handlePointerMove,
       onPointerUp: controller.handlePointerUp,
       onPointerCancel: controller.handlePointerCancel
     );
   }
 
 }