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)
);