Correct bottom overscroll in VariableHeightScrollable

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 'package:newton/newton.dart';
 import 'package:sky/animation/animated_simulation.dart';
 import 'package:sky/animation/scroll_behavior.dart';
 import 'package:sky/theme/view_configuration.dart' as config;
 import 'package:sky/widgets/basic.dart';
 
 const double _kMillisecondsPerSecond = 1000.0;
 
 double _velocityForFlingGesture(double eventVelocity) {
+  // eventVelocity is pixels/second, config min,max limits are pixels/ms
   return eventVelocity.clamp(-config.kMaxFlingVelocity, config.kMaxFlingVelocity) /
     _kMillisecondsPerSecond;
 }
 
 abstract class ScrollClient {
   bool ancestorScrolled(Scrollable ancestor);
 }
 
 enum ScrollDirection { vertical, horizontal }
 
@@ skipping 133 matching lines @@
 
   void _handleFlingCancel(sky.GestureEvent event) {
     settleScrollOffset();
   }
 
   void _handleWheel(sky.WheelEvent event) {
     scrollBy(-event.offsetY);
   }
 
 }