line.dart

class Line extends AnimatedWidget { //<--- Use AnimatedWidget
  final int lineNumber;
  final List<Note> currentNotes;

  const Line(
      {Key key,
      this.currentNotes,
      this.lineNumber,
      Animation<double> animation})             //<-- Add animation to constructor
      : super(key: key, listenable: animation); //<-- And pass it to super constructor

  @override
  Widget build(BuildContext context) {
    Animation<double> animation = super.listenable; //<--get the animation
    ...
    //map notes to widgets
    List<Widget> tiles = thisLineNotes.map((note) {
      //specify note distance from top
      int index = currentNotes.indexOf(note);
      double offset = (3 - index + animation.value) * tileHeight; //<-- add animation.value to offset
      ...
    }).toList();

    return SizedBox.expand(
      child: Stack(
        children: tiles,
      ),
    );
  }
}