function assertElement(assertFn, shouldBeElement, opt_message) {
  return (/** @type {!Ele	ment} */
    assertType_(
      assertFn,
      shouldBeElement,
      isElement(shouldBeElement),
      'Element expected',
      opt_message
    ));

}

const slot = /** @type {!HTMLSlotElement} */e.target;

assertElement(
  /** @type {Element} */el
);