Port PopupMenu to the 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 {
@@ skipping 42 matching lines @@
   double get progress => timeline.value;
   void set progress(double t) {
     stop();
     timeline.value = t.clamp(0.0, 1.0);
   }
 
   bool get isDismissed => progress == 0.0;
   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() {
     timeline.stop();
   }
 
   // Resume animating in a direction, with the given velocity.
   // TODO(mpcomplete): this should be a force with friction so it slows over
   // time.
   void fling({double velocity: 1.0}) {
@@ skipping 14 matching lines @@
   void removeListener(Function listener) {
     _listeners.remove(listener);
   }
 
   void _notifyListeners() {
     List<Function> localListeners = new List<Function>.from(_listeners);
     for (Function listener in localListeners)
       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) {
     variable.setFraction(t);
     _notifyListeners();
   }
 }