Refactor fn2.dart, since it breached our 1000-line threshold.

sky/sdk/lib/framework/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 '../animation/scroll_behavior.dart';
-import '../fn2.dart';
-import 'dart:async';
-import 'dart:math' as math;
-import 'package:vector_math/vector_math.dart';
 import 'scrollable.dart';
+import 'wrappers.dart';
 
 abstract class FixedHeightScrollable extends Scrollable {
 
   FixedHeightScrollable({ this.itemHeight, Object key }) : super(key: key) {
     assert(itemHeight != null);
   }
 
   double itemHeight;
 
   void syncFields(FixedHeightScrollable source) {
@@ skipping 57 matching lines @@
             child: new Block(buildItems(itemShowIndex, itemShowCount))
           )
         )
       )
     );
   }
 
   List<UINode> buildItems(int start, int count);  
 
 }