Check if wp_nav_menu items have submenus

If I understand correctly, you want links with submenus to have an attribute of aria-haspopup. If this is correct, you should be able to do so using the nav_menu_link_attributes filter (WP 3.6 and above). You can also get around the necessity of having to write a custom Walker to check if an item has children by checking its css classes in the filter.

add_filter( 'nav_menu_link_attributes', 'wpse154485_add_aria_haspopup_atts', 10, 3 );
function wpse154485_add_aria_haspopup_atts( $atts, $item, $args ) {
  if (in_array('menu-item-has-children', $item->classes)) {
    $atts['aria-haspopup'] = 'true';
  }
  return $atts;
}