[Effen] fix some dart warnings (mostly unused imports)

sky/framework/components/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 '../animation/scroll_behavior.dart';
 import '../debug/tracing.dart';
 import '../fn.dart';
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 import 'dart:async';
 import 'scrollable.dart';
-import '../layouts/block.dart';
 
 abstract class FixedHeightScrollable extends Scrollable {
   static final Style _style = new Style('''
     overflow: hidden;
     position: relative;
     will-change: transform;'''
   );
 
   static final Style _scrollAreaStyle = new Style('''
     position:relative;
     will-change: transform;'''
   );
 
   FixedHeightScrollable({
     Object key
   }) : super(key: key);
 
   ScrollBehavior createScrollBehavior() => new OverscrollBehavior();
+  OverscrollBehavior get scrollBehavior => super.scrollBehavior as OverscrollBehavior;
 
   double _height = 0.0;
   double _itemHeight;
 
   int _itemCount = 0;
   int get itemCount => _itemCount;
   void set itemCount (int value) {
     if (_itemCount != value) {
       _itemCount = value;
       if (_itemHeight != null)
@@ skipping 58 matching lines @@
       style: _style,
       children: [
         new Container(
           style: _scrollAreaStyle,
           inlineStyle: transformStyle,
           children: buildItems(itemNumber, drawCount)
         )
       ]
     );
   }
+
+  List<UINode> buildItems(int start, int count);  
 }