Scrollable should settle back to 0.0

sky/framework/animation/generator.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 'curves.dart';
 import 'dart:async';
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 
 class FrameGenerator {
@@ skipping 43 matching lines @@
 class AnimationGenerator extends FrameGenerator {
   Stream<double> get onTick => _stream;
   final double initialDelay;
   final double duration;
   final double begin;
   final double end;
   final Curve curve;
   Stream<double> _stream;
   bool _done = false;
 
-  AnimationGenerator(this.duration, {
+  AnimationGenerator({
     this.initialDelay: 0.0,
+    this.duration,
     this.begin: 0.0,
     this.end: 1.0,
     this.curve: linear,
     Function onDone
   }):super(onDone: onDone) {
-    assert(duration > 0);
+    assert(duration != null && duration > 0.0);
     double startTime = 0.0;
     _stream = super.onTick.map((timeStamp) {
       if (startTime == 0.0)
         startTime = timeStamp;
 
       double t = (timeStamp - (startTime + initialDelay)) / duration;
       return math.max(0.0, math.min(t, 1.0));
     })
     .takeWhile(_checkForCompletion) //
     .where((t) => t >= 0.0)
@@ skipping 43 matching lines @@
     if (_animation != null) {
       _animation.cancel();
       _animation = null;
     }
   }
 
   void animateTo(double newValue, double duration,
       { Curve curve: linear, double initialDelay: 0.0 }) {
     stop();
 
-    _animation = new AnimationGenerator(duration, begin: _value, end: newValue,
-        curve: curve, initialDelay: initialDelay);
+    _animation = new AnimationGenerator(
+        duration: duration,
+        begin: _value,
+        end: newValue,
+        curve: curve,
+        initialDelay: initialDelay);
 
     _animation.onTick.listen(_setValue, onDone: () {
       _animation = null;
     });
   }
 }