testDownKey() {
    popup.decorate(menu);
    popup.attach(anchor);
    popup.hide();
    assertFalse(popup.isVisible());
    events.fireKeySequence(anchor, KeyCodes.DOWN);
    assertTrue(popup.isVisible());
    assertEquals(0, popup.getHighlightedIndex());
  }
Tests that the menu is shown and the first item is highlighted if the DOWN
key is pressed.