Port PopupMenu to the new animation system

sky/sdk/lib/animation/timeline.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 'dart:async';
 
 import '../base/scheduler.dart' as scheduler;
 
 class Ticker {
   Ticker(Function onTick) : _onTick = onTick;
@@ skipping 62 matching lines @@
   void set value(double newValue) {
     assert(newValue != null && newValue >= 0.0 && newValue <= 1.0);
     assert(!_ticker.isTicking);
     _value = newValue;
     _onTick(_value);
   }
 
   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;
 
     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();
   }
 
   bool get isAnimating => _ticker.isTicking;
 
   void _tick(double timeStamp) {
     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;
     _onTick(_value);
 
     if (isLastTick)
       stop();
   }
 }