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) {
  double turnsValue = turns.value;
  Matrix4 transform = new Matrix4.rotationZ(turnsValue * math.PI * 2.0);
  return new Transform(
    transform: transform,
    alignment: FractionalOffset.center,
    child: child
  );
}