Refactor stateful parts of Component into StatefulComponent

sky/sdk/lib/widgets/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:sky' as sky;
 
 import '../animation/generators.dart';
 import '../animation/mechanics.dart';
 import '../animation/scroll_behavior.dart';
 import '../theme/view_configuration.dart' as config;
 import 'basic.dart';
 import 'material.dart';
 
 const double _kMillisecondsPerSecond = 1000.0;
 
 double _velocityForFlingGesture(double eventVelocity) {
   return eventVelocity.clamp(-config.kMaxFlingVelocity, config.kMaxFlingVelocity) /
     _kMillisecondsPerSecond;
 }
 
 abstract class ScrollClient {
   bool ancestorScrolled(Scrollable ancestor);
 }
 
 enum ScrollDirection { vertical, horizontal }
 
-abstract class Scrollable extends Component {
+abstract class Scrollable extends StatefulComponent {
 
   Scrollable({
    String key,
    this.backgroundColor,
    this.direction: ScrollDirection.vertical
-  }) : super(key: key, stateful: true);
+  }) : super(key: key);
 
   Color backgroundColor;
   ScrollDirection direction;
 
   void syncFields(Scrollable source) {
     backgroundColor = source.backgroundColor;
     direction == source.direction;
   }
 
   double _scrollOffset = 0.0;
@@ skipping 119 matching lines @@
 
   void _handleFlingCancel(sky.GestureEvent event) {
     _startSimulation(_createParticle());
   }
 
   void _handleWheel(sky.WheelEvent event) {
     scrollBy(-event.offsetY);
   }
 
 }