1. override
Widget build(BuildContext context)

Override this method to build widgets that depend on the current value of the animation.

Source 

@override
Widget build(BuildContext context) {
  final double scaleValue = scale.value;
  Matrix4 transform = new Matrix4.identity()
    ..scale(scaleValue, scaleValue, 1.0);
  return new Transform(
    transform: transform,
    alignment: alignment,
    child: child
  );
}