Remove redundant setState() calls around scroll behaviour updates.

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';
 
@@ skipping 67 matching lines @@
         double drawStart = scrollOffset + alignmentDelta;
         itemShowIndex = math.max(0, (drawStart / itemHeight).floor());
 
         offsetY = -alignmentDelta;
       }
     }
 
     List<Widget> items = buildItems(itemShowIndex, itemShowCount);
     assert(items.every((item) => item.key != null));
 
+    // TODO(ianh): Refactor this so that it does the building in the
+    // same frame as the size observing, similar to BlockViewport, but
+    // keeping the fixed-height optimisations.
     return new SizeObserver(
       callback: _handleSizeChanged,
       child: new Viewport(
         offset: offsetY,
         child: new Container(
           padding: padding,
           child: new Block(items)
         )
       )
     );
   }
 
   List<Widget> buildItems(int start, int count);
 
 }