code: function setPhase(priv, phase) {
    check(phase, 'phase').is.oneOf(PHASE_VALUES)();
    if (priv.phase !== null) {
      priv.elem.classList.remove(priv.phase);
    }
    priv.phase = phase;
  
    if (phase !== null) {
      priv.elem.classList.add(phase);
    }
    priv.listeners.forEach(function(listener) {
      listener(phase);
    });
    maybeStart(priv);
  }
docs: Changes current phase.
  
  Invoking this method will result in setting CSS class name
  of requested phase on container element.
  
  @param {String} phase desired phase
  @fqn Phaser.prototype.setPhase