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);