More minor cleanup around overriding fields to narrow types.

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({ 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);
   }
 
   ScrollBehavior createScrollBehavior() => new OverscrollBehavior();
-  OverscrollBehavior get scrollBehavior => super.scrollBehavior as OverscrollBehavior;
+  OverscrollBehavior get scrollBehavior => super.scrollBehavior;
 
   int _itemCount = 0;
   int get itemCount => _itemCount;
   void set itemCount (int value) {
     if (_itemCount != value) {
       _itemCount = value;
       scrollBehavior.contentsHeight = itemHeight * _itemCount;
     }
   }
 
@@ skipping 36 matching lines @@
           transform: transform,
           child: new Block(buildItems(itemShowIndex, itemShowCount))
         )
       )
     );
   }
 
   List<UINode> buildItems(int start, int count);
 
 }