constructor(type, mechanism, cell, key) {
    /**
     * @private {string}
     * @const
     */
    this.type_ = type;
    /**
     * @private {string}
     * @const
     */
    this.mechanism_ = mechanism;
    /**
     * @private {string}
     * @const
     */
    this.cell_ = cell;
    /**
     * @private {number}
     * @const
     */
    this.key_ = key;

    /**
     * @private {string}
     * @const
     */
    this.asString_ = [
      'offline:', type, '/', mechanism, '/', cell, '/', key,
    ].join('');
  }
@param {string} type
@param {string} mechanism
@param {string} cell
@param {number} key