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