/// The [tween] also defines the target value for the animation: When the widget
/// first builds, it animates from [Tween.begin] to [Tween.end]. A new animation
/// can be triggered anytime by providing a new [tween] with a new [Tween.end]
/// value. The new animation runs from the current animation value (which may be
/// [Tween.end] of the old [tween], if that animation completed) to [Tween.end]
/// of the new [tween].