constructor(key, defaultState) {
    /**
     * @type {string}
     * @private
     */
    this.key_ = key;

    /**
     * @type {!Object}
     * @private
     */
    this.state_ = Object.create(null);

    /**
     * @type {!Object}
     * @private
     */
    this.defaultState_ = defaultState || Object.create(null);
  }
Local storage state constructor.
@param {string} key The local storage key.
@param {Object=} defaultState The default state.