anchor.diff:  
       Sound laserSound = new Sound(this, "laser");
   
  -
  -    const spaceBar = 32;
  -    const leftArrow = 37;
  -    const rightArrow = 39;
  -
  -
       Sprite bullet;
       Timer bulletTimer = new Timer.periodic(new Duration(milliseconds: 450), (_)
           {
  
positive.code: 
      game.stage.onKeyDown.listen((value) {
        switch (value.keyCode) {
          case leftArrow:
            player.movingLeft = true;
            break;
          case rightArrow:
            player.movingRight = true;
            break;
        }
  
negative.code:     Text scoreText = new Text(this, "Score: $score", size: 20);
  
      Button leftButton = new Button(this, "leftButton", "L");
      leftButton.setCoordinates(0, game.stage.sourceHeight - 100);
  
      Button rightButton = new Button(this, "rightButton", "R");
      rightButton.setCoordinates(game.stage.sourceWidth -
          rightButton.width.toInt(), game.stage.sourceHeight - 100);
  
      Ship player = new Ship(this, "ship")
          ..x = game.stage.sourceWidth ~/ 2
          ..y = game.stage.sourceHeight - 200
          ..vx = 300;
  
      leftButton.onTouchBegin.listen((_) {
  