getScaleAnimation method

Animates the scale of the FloatingActionButton.

The animation should both start and end with a value of 1.0.

For example, to create an animation that linearly scales out and then back in, you could join animations that pass each other:

  @override
  Animation<double> getScaleAnimation({@required Animation<double> parent}) {
    // The animations will cross at value 0, and the train will return to 1.0.
    return TrainHoppingAnimation(
      Tween<double>(begin: 1.0, end: -1.0).animate(parent),
      Tween<double>(begin: -1.0, end: 1.0).animate(parent),
    );
  }