Make the popup menu width animation work

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:sky/animation/scroll_behavior.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/scrollable.dart';
 
 abstract class FixedHeightScrollable extends Scrollable {
 
-  FixedHeightScrollable({ String key, this.itemHeight, Color backgroundColor, this.padding })
-      : super(key: key, backgroundColor: backgroundColor) {
+  FixedHeightScrollable({ String key, this.itemHeight, this.padding })
+      : super(key: key) {
     assert(itemHeight != null);
   }
 
   EdgeDims padding;
   double itemHeight;
 
   /// Subclasses must implement `get itemCount` to tell FixedHeightScrollable
   /// how many items there are in the list.
   int get itemCount;
   int _previousItemCount;
@@ skipping 72 matching lines @@
           padding: padding,
           child: new Block(items)
         )
       )
     );
   }
 
   List<Widget> buildItems(int start, int count);
 
 }