Polish the menu animation

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 9f13a3553aebcb1429df3a41bf698aa7cc247f43..11c9822d71b44c716c0a772a448fc92af8f8e4fa 100644
--- a/sky/sdk/lib/animation/animation_performance.dart
+++ b/sky/sdk/lib/animation/animation_performance.dart
@@ -54,11 +54,16 @@ class AnimatedType<T extends dynamic> extends AnimatedVariable {
 
 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])';
 }