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);