createTween method
Returns a Tween that the AnimationImpl can use to interpolate between
value and animatedValue.
The returned Tween must always evaluate to T even though the signature
allows for a nullable type. The signature is chosen to allow the usage of
Tweens that only return null when neither Tween.begin nor
Tween.end are set, which AnimationImpl will never do.
Implementation
@override
Tween<int?> createTween() => IntTween();