[Effen] Prevent scrolling past the bottom of a scrollable list.

sky/framework/components/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/generators.dart';
 import '../animation/mechanics.dart';
 import '../animation/scroll_behavior.dart';
 import '../fn.dart';
 import '../theme/view-configuration.dart' as config;
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 
 const double _kMillisecondsPerSecond = 1000.0;
 
 double _velocityForFlingGesture(sky.GestureEvent event) {
   return math.max(-config.kMaxFlingVelocity, math.min(config.kMaxFlingVelocity,
       -event.velocityY)) / _kMillisecondsPerSecond;
 }
 
 abstract class ScrollClient {
   bool ancestorScrolled(Scrollable ancestor);
 }
 
 abstract class Scrollable extends Component {
-  ScrollBehavior scrollBehavior;
+
+  double _scrollOffset = 0.0;
   double get scrollOffset => _scrollOffset;
 
-  double _scrollOffset = 0.0;
+  ScrollBehavior _scrollBehavior;
+  ScrollBehavior createScrollBehavior();
+  ScrollBehavior get scrollBehavior {
+    if (_scrollBehavior == null)
+      _scrollBehavior = createScrollBehavior();
+    return _scrollBehavior;
+  }
+
   Simulation _simulation;
 
-  Scrollable({Object key, this.scrollBehavior}) : super(key: key) {
+  Scrollable({Object key}) : super(key: key) {
     onDidUnmount(_stopSimulation);
   }
 
   UINode buildContent();
 
   UINode build() {
     return new EventListenerNode(
       buildContent(),
       onPointerDown: _handlePointerDown,
       onPointerUp: _handlePointerUpOrCancel,
@@ skipping 87 matching lines @@
   }
 
   void _handleFlingCancel(sky.GestureEvent event) {
     _startSimulation(_createParticle());
   }
 
   void _handleWheel(sky.WheelEvent event) {
     scrollBy(-event.offsetY);
   }
 }