/**
  * {@inheritdoc}
  */
 protected function prepareMigration(MigrationInterface $migration)
 {
     // File migrations need a source_base_path.
     // @see MigrateUpgradeRunBatch::run
     $destination = $migration->getDestinationConfiguration();
     if ($destination['plugin'] === 'entity:file') {
         // Make sure we have a single trailing slash.
         $source = $migration->getSourceConfiguration();
         $source['site_path'] = 'core/modules/simpletest';
         $source['constants']['source_base_path'] = \Drupal::root() . '/';
         $migration->set('source', $source);
     }
 }