Solution: Beating Heart

// SOLUTION #1 - add TickerProviderStateMixin to the widget state class
class _ContactListScreenState extends State<ContactListScreen> with TickerProviderStateMixin {
    //...
// SOLUTION #2 Create an animation controller
  late final AnimationController _controller = AnimationController(
    duration: const Duration(milliseconds: 500),
    vsync: this,
  )..repeat(reverse: true);
 
// SOLUTION #3 Create an animation that uses the animation controller
  late final Animation<double> _animation =
    Tween<double>(begin: 16, end: 24).animate(_controller);
 
// SOLUTION #4 - Use an animated builder to animate the heart icon
// ...
AnimatedBuilder(
    animation: _animation,
    builder: (context, child) {
        return Icon(
            Icons.favorite,
            color: Colors.red,
            size: _animation.value,
        );
    },
),
// SOLUTION #5 - dispose the animation controller on widget dispose
    _controller.dispose();