function assertArrowDirection(subMenu, left) {
  assertEquals(
      left ? SubMenuRenderer.LEFT_ARROW_ : SubMenuRenderer.RIGHT_ARROW_,
      getArrowElement(subMenu).innerHTML);
}
Asserts that this sub menu has a properly-oriented arrow.
@param {SubMenu} subMenu The sub menu.
@param {boolean} left True for left-pointing, false for right-pointing.
@suppress {visibility} suppression added to enable type checking