fix the hang when clicking the drawer in the stocks app

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 48 matching lines @@
   final Curve curve;
   Stream<double> _stream;
   bool _done = false;
 
   AnimationGenerator(this.duration, {
     this.begin: 0.0,
     this.end: 1.0,
     this.curve: linear,
     Function onDone
   }):super(onDone: onDone) {
+    assert(duration > 0);
     double startTime = 0.0;
     _stream = super.onTick.map((timeStamp) {
       if (startTime == 0.0)
         startTime = timeStamp;
       return math.min((timeStamp - startTime) / duration, 1.0);
     })
     .takeWhile(_checkForCompletion)
     .map((t) =>  begin + (end - begin) * curve.transform(t));
   }
 
   bool _checkForCompletion(double t) {
     if (_done)
       return false;
     _done = t >= 1;
     return true;
   }
 }