code: function addPhaseListener(priv, listener) {
    priv.listeners.push(check(listener, 'listener').is.aFunction());
  }
docs: Adds a listener that will be notified on phase changes.
  
  It is used by the ${link Slider} to change styles of dots representing slides.
  
  @param {Function} listener listener to be added
  
  @fqn Phaser.prototype.addPhaseListener