Add slug to css classes in wp_nav_menu() menu items. For use in /wp-content/themes/myTheme/functions.php.

add_slug_class_to_menu_item.php

/**
 * Add slugs to custom menu items
 */
function add_slug_class_to_menu_item($output){
  $ps = get_option('permalink_structure');
  if(!empty($ps)){
    $idstr = preg_match_all('/<li id="menu-item-(\d+)/', $output, $matches);
    foreach($matches[1] as $mid){
      $id = get_post_meta($mid, '_menu_item_object_id', true);
      $slug = basename(get_permalink($id));
      $output = preg_replace('/menu-item-'.$mid.'">/', 'menu-item-'.$mid.' menu-item-'.$slug.'">', $output, 1);
    }
  }
  return $output;
}
add_filter('wp_nav_menu', 'add_slug_class_to_menu_item');