Animation constructor

Animation([AnimationEffectReadOnly effect, AnimationTimeline timeline ])

Implementation 

factory Animation(
    [AnimationEffectReadOnly effect, AnimationTimeline timeline]) {
  if (timeline != null) {
    return Animation._create_1(effect, timeline);
  }
  if (effect != null) {
    return Animation._create_2(effect);
  }
  return Animation._create_3();
}