code: function addMirrorClasses({mirror, mirrorClass, ...args}) {
    return withPromise((resolve) => {
      mirror.classList.add(mirrorClass);
      resolve({mirror, mirrorClass, ...args});
    });
  }
docs: Applys mirror class on mirror element
  @param {Object} state
  @param {HTMLElement} state.mirror
  @param {String} state.mirrorClass
  @return {Promise}
  @private