onComplete fires before onPlay on zero duration animations

[gskinner]

For example, this:

.animate(
   onInit: (controller) => print('init'),
   onPlay: (controller) => print('play'),
   onComplete: (controller) => print('complete'),
)
.slideY(duration: 0.ms)

Will print:

flutter: init
flutter: complete
flutter: play

This is because we need to fire onPlay after the animation is started (so it can be stopped in the onPlay), but a zero duration controller instantly updates it's status as completed.