Port PopupMenu to the new animation system

sky/sdk/lib/animation/timeline.dart

Index: sky/sdk/lib/animation/timeline.dart
diff --git a/sky/sdk/lib/animation/timeline.dart b/sky/sdk/lib/animation/timeline.dart
index 0d5f3682bf3fa59af5caf6b54e056749a039add2..8b0b43f1dfcaa55cf947e6ece0fe891dd6ac2640 100644
--- a/sky/sdk/lib/animation/timeline.dart
+++ b/sky/sdk/lib/animation/timeline.dart
@@ -80,23 +80,27 @@ class Timeline {
   double _duration;
   double _begin;
   double _end;
+  double _delay;
 
   double _startTime;
 
   Future start({
     double duration,
     double begin: 0.0,
-    double end: 1.0
+    double end: 1.0,
+    double delay: 0.0
   }) {
     assert(duration != null && duration > 0.0);
     assert(begin != null && begin >= 0.0 && begin <= 1.0);
     assert(end != null && end >= 0.0 && end <= 1.0);
+    assert(delay != null && delay >= 0.0);
 
     assert(!_ticker.isTicking);
 
     _duration = duration;
     _begin = begin;
     _end = end;
+    _delay = delay;
 
     _startTime = null;
     _value = begin;
@@ -104,14 +108,15 @@ class Timeline {
     return _ticker.start();
   }
 
-  Future animateTo(double target, { double duration }) {
-    return start(duration: duration, begin: _value, end: target);
+  Future animateTo(double target, { double duration, double delay }) {
+    return start(duration: duration, begin: _value, end: target, delay: delay);
   }
 
   void stop() {
     _duration = null;
     _begin = null;
     _end = null;
+    _delay = null;
     _startTime = null;
     _ticker.stop();
   }
@@ -122,7 +127,7 @@ class Timeline {
     if (_startTime == null)
       _startTime = timeStamp;
 
-    final double t = ((timeStamp - _startTime) / _duration).clamp(0.0, 1.0);
+    final double t = ((timeStamp - (_startTime + _delay)) / _duration).clamp(0.0, 1.0);
     final bool isLastTick = t >= 1.0;
 
     _value = isLastTick ? _end : _begin + (_end - _begin) * t;