testKeyboardEventsShowMenu() {
    popup.decorate(menu);
    popup.attach(anchor);
    popup.hide();
    assertFalse(popup.isVisible());
    events.fireKeySequence(anchor, KeyCodes.SPACE);
    assertTrue(popup.isVisible());
    assertEquals(-1, popup.getHighlightedIndex());
    popup.hide();
    assertFalse(popup.isVisible());
    events.fireKeySequence(anchor, KeyCodes.ENTER);
    assertTrue(popup.isVisible());
    assertEquals(-1, popup.getHighlightedIndex());
  }
Tests that the menu is shown if the SPACE or ENTER keys are pressed, and
that none of the menu items are highlighted (PopupMenu.highlightedIndex ==
-1).