onXHRLoad_(event) {
    const client = /** @type {XMLHttpRequest} */ (event.target);
    // status will be 0 for file:// urls
    if (!client.status || (client.status >= 200 && client.status < 300)) {
      let response;
      try {
        response = /** @type {Config} */ (JSON.parse(client.responseText));
      } catch {
        this.handleTileJSONError();
        return;
      }
      this.handleTileJSONResponse(response);
    } else {
      this.handleTileJSONError();
    }
  }
@private
@param {Event} event The load event.