constructor() {
    /**
     * @type {string} The block xml to use for test. Do not provide if json is
     *     provided.
     */
    this.xml;
    /**
     * @type {string|undefined} The expected xml after round trip. Provided if
     *    it different from xml that was passed in.
     */
    this.expectedXml;
    /**
     * @type {string} The block json to use for test. Do not provide if xml is
     *     provided.
     */
    this.json;
    /**
     * @type {string|undefined} The expected json after round trip. Provided if
     *    it is different from json that was passed in.
     */
    this.expectedJson;
  }
Class for a block serialization test case.