Make UINode's key a String

sky/sdk/lib/widgets/fixed_height_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 'package:vector_math/vector_math.dart';
 
 import '../framework/animation/scroll_behavior.dart';
 import 'basic.dart';
 import 'scrollable.dart';
 
 abstract class FixedHeightScrollable extends Scrollable {
 
-  FixedHeightScrollable({ this.itemHeight, Color backgroundColor, Object key })
+  FixedHeightScrollable({ String key, this.itemHeight, Color backgroundColor })
       : super(key: key, backgroundColor: backgroundColor) {
     assert(itemHeight != null);
   }
 
   double itemHeight;
 
   void syncFields(FixedHeightScrollable source) {
     itemHeight = source.itemHeight;
     super.syncFields(source);
   }
@@ skipping 49 matching lines @@
           transform: transform,
           child: new Block(buildItems(itemShowIndex, itemShowCount))
         )
       )
     );
   }
 
   List<UINode> buildItems(int start, int count);
 
 }