Update ink splash to new animation system

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 'timeline.dart';
 import 'curves.dart';
 
 // TODO(mpcomplete): merge this stuff with AnimatedValue somehow. We shouldn't
 // have 2 different ways to animate values.
 abstract class AnimatedVariable {
   void setFraction(double t);
 }
 
 class AnimatedType<T extends dynamic> extends AnimatedVariable {
-  T value;
-  T begin, end;
-  final Curve curve;
+  AnimatedType(this.begin, {this.end, this.curve: linear}) {
+    _value = begin;
+  }
 
-  AnimatedType(this.begin, {this.end, this.curve: linear}) {
-    value = begin;
-  }
+  T begin;
+  T end;
+  Curve curve;
+
+  T _value;
+  T get value => _value;
 
   void setFraction(double t) {
     if (end != null) {
       // TODO(mpcomplete): Reverse the timeline and curve.
-      value = begin + (end - begin) * curve.transform(t);
+      _value = begin + (end - begin) * curve.transform(t);
     }
   }
 }
 
 // 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
@@ skipping 66 matching lines @@
     timeline.stop();
     if (remainingDistance != 0.0)
       timeline.animateTo(target, duration: remainingDistance * duration.inMilliseconds);
   }
 
   void _tick(double t) {
     variable.setFraction(t);
     _notifyListeners();
   }
 }