constructor(tileUrl, opt_errorMessage) {
    super(ee.layers.AbstractOverlay.EventType.TILE_FAIL);

    /** @const {string} The URL of the failed tile. */
    this.tileUrl = tileUrl;

    /**
     * A message describing the error that caused the tile to fail, if any.
     * @const {string|undefined}
     */
    this.errorMessage = opt_errorMessage;
  }
@param {string} tileUrl The URL of the failed tile.
@param {string=} opt_errorMessage A message describing the error that
    caused the tile to fail, if any.