code: function focusMenuItem(e, menuEl, opts, direction) {
        var currentItem = $mdUtil.getClosest(e.target, 'MD-MENU-ITEM');
  
        var items = $mdUtil.nodesToArray(menuEl[0].children);
        var currentIndex = items.indexOf(currentItem);
  
        // Traverse through our elements in the specified direction (+/-1) and try to
        // focus them until we find one that accepts focus
        var didFocus;
        for (var i = currentIndex + direction; i >= 0 && i < items.length; i = i + direction) {
          var focusTarget = items[i].querySelector('.md-button');
          didFocus = attemptFocus(focusTarget);
          if (didFocus) {
            break;
          }
        }
        return didFocus;
      }
docs: Takes a keypress event and focuses the next/previous menu
  item from the emitting element
  @param {event} e - The origin keypress event
  @param {angular.element} menuEl - The menu element
  @param {object} opts - The interim element options for the mdMenu
  @param {number} direction - The direction to move in (+1 = next, -1 = prev)