Update ink splash to new animation system

sky/sdk/lib/animation/animation_performance.dart

Index: sky/sdk/lib/animation/animation_performance.dart
diff --git a/sky/sdk/lib/animation/animation_performance.dart b/sky/sdk/lib/animation/animation_performance.dart
index 829f9e55ad8ddba46d3df9965d73bc4e0923db21..a9e89af7523a5f050e0c08026acdce414672a90d 100644
--- a/sky/sdk/lib/animation/animation_performance.dart
+++ b/sky/sdk/lib/animation/animation_performance.dart
@@ -12,18 +12,21 @@ abstract class AnimatedVariable {
 }
 
 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;
+    _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);
     }
   }
 }