Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / web / core / modules / rdf / tests / rdf_test_namespaces / rdf_test_namespaces.module

<?php

/**
 * @file
 * Test the namespace registration functionality.
 */

/**
 * Implements hook_rdf_namespaces().
 */
function rdf_test_namespaces_rdf_namespaces() {
  return [
    'foaf'     => 'http://xmlns.com/foaf/0.1/',
    'foaf1'    => 'http://xmlns.com/foaf/0.1/',
  ];
}