Polish the menu animation

sky/sdk/lib/animation/animation_performance.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 'package:sky/animation/timeline.dart';
 import 'package:sky/animation/curves.dart';
 
 abstract class AnimatedVariable {
   void setFraction(double t);
   String toString();
@@ skipping 36 matching lines @@
         value = begin + (end - begin) * curve.transform(adjustedTime);
       }
     }
   }
 
   String toString() => 'AnimatedType(begin=$begin, end=$end, value=$value)';
 }
 
 class AnimatedList extends AnimatedVariable {
   List<AnimatedVariable> variables;
-  AnimatedList(this.variables);
+  Interval interval;
+
+  AnimatedList(this.variables, { this.interval });
+
   void setFraction(double t) {
+    double adjustedTime = interval == null ? t : interval.adjustTime(t);
     for (AnimatedVariable variable in variables)
-      variable.setFraction(t);
+      variable.setFraction(adjustedTime);
   }
+
   String toString() => 'AnimatedList([$variables])';
 }
 
 // This class manages a "performance" - a collection of values that change
 // based on a timeline. For example, a performance may handle an animation
 // of a menu opening by sliding and fading in (changing Y value and opacity)
 // over .5 seconds. The performance can move forwards (present) or backwards
 // (dismiss). A consumer may also take direct control of the timeline by
 // manipulating |progress|, or |fling| the timeline causing a physics-based
 // simulation to take over the progression.
@@ skipping 65 matching lines @@
     timeline.stop();
     if (remainingDistance != 0.0)
       timeline.animateTo(target, duration: remainingDistance * duration.inMilliseconds);
   }
 
   void _tick(double t) {
     variable.setFraction(t);
     _notifyListeners();
   }
 }