createSingleTween method
Creates a single Animatable from from to to values.
Defaults to Tween. Subclasses can override to use specialized animatables (e.g., ColorTween, SizeTween).
Implementation
@override
Animatable<Stretch> createSingleTween(Stretch from, Stretch to) {
return _StretchTween(begin: from, end: to);
}