Don't use globals

[curtismchale]

@lukefretwell we shouldn't be using globals like is done here.

The action hook passes the admin menu value you use. When you use a global I can't edit the menu stuff again later with a custom plugin without messing with load orders.

add_action( 'admin_bar_menu', 'lukes_admin_bar' );
function lukes_admin_bar( $admin_bar ){

  $args = array(
        'id' => 'media-library',
        'title' => 'Media Library', 
        'href' => esc_url( admin_url( 'upload.php' ) )
    );
    $admin_bar->add_node($args);

}

This is how it should be done. In general if you think you should use global you should not.