Remove redundant setState() calls around scroll behaviour updates.

sky/sdk/lib/widgets/scrollable_viewport.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 'package:sky/animation/scroll_behavior.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/scrollable.dart';
 
 class ScrollableViewport extends Scrollable {
 
   ScrollableViewport({ String key, this.child }) : super(key: key);
 
   Widget child;
 
   void syncFields(ScrollableViewport source) {
     child = source.child;
     super.syncFields(source);
   }
 
   ScrollBehavior createScrollBehavior() => new FlingBehavior();
   FlingBehavior get scrollBehavior => super.scrollBehavior;
 
   double _viewportHeight = 0.0;
   double _childHeight = 0.0;
   void _handleViewportSizeChanged(Size newSize) {
-    setState(() {
-      _viewportHeight = newSize.height;
-      _updateScrollBehaviour();
-    });
+    _viewportHeight = newSize.height;
+    _updateScrollBehaviour();
   }
   void _handleChildSizeChanged(Size newSize) {
-    setState(() {
-      _childHeight = newSize.height;
-      _updateScrollBehaviour();
-    });
+    _childHeight = newSize.height;
+    _updateScrollBehaviour();
   }
   void _updateScrollBehaviour() {
     scrollBehavior.contentsSize = _childHeight;
     scrollBehavior.containerSize = _viewportHeight;
     if (scrollOffset > scrollBehavior.maxScrollOffset)
       settleScrollOffset();
   }
 
   Widget buildContent() {
     return new SizeObserver(
@@ skipping 16 matching lines @@
 
   final List<Widget> children;
 
   Widget build() {
     return new ScrollableViewport(
       child: new Block(children)
     );
   }
 
 }