Port PopupMenu to the 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 d6d9edc1f8fc268c630c6fd64fb9fab5f85b5bf4..50f9c8a2368a994553009f846ed62f0701466cc7 100644
--- a/sky/sdk/lib/animation/animation_performance.dart
+++ b/sky/sdk/lib/animation/animation_performance.dart
@@ -60,11 +60,11 @@ class AnimationPerformance {
   bool get isCompleted => progress == 1.0;
   bool get isAnimating => timeline.isAnimating;
 
-  void play() {
-    _animateTo(1.0);
+  void play({ Duration delay: Duration.ZERO }) {
+    _animateTo(1.0, delay: delay);
   }
-  void reverse() {
-    _animateTo(0.0);
+  void reverse({ Duration delay: Duration.ZERO }) {
+    _animateTo(0.0, delay: delay);
   }
 
   void stop() {
@@ -99,11 +99,14 @@ class AnimationPerformance {
       listener();
   }
 
-  void _animateTo(double target) {
+  void _animateTo(double target, { Duration delay }) {
+    assert(delay != null);
     double remainingDistance = (target - timeline.value).abs();
     timeline.stop();
-    if (remainingDistance != 0.0)
-      timeline.animateTo(target, duration: remainingDistance * duration.inMilliseconds);
+    if (remainingDistance != 0.0) {
+      timeline.animateTo(target, duration: remainingDistance * duration.inMilliseconds,
+                                 delay: delay.inMilliseconds.toDouble());
+    }
   }
 
   void _tick(double t) {