prepareMigrations example

'created' => 1,
        'changed' => 1,
      ]);
      $file->setPermanent();
      $file->enforceIsNew();
      file_put_contents($file->getFileUri(), 'hello world');

      // Save it, inserting a new record.       $file->save();
      $id_mappings['d6_file'][] = [[$i], [$i]];
    }
    $this->prepareMigrations($id_mappings);

    $this->migrateContent(['translations']);
    // Since we are only testing a subset of the file migration, do not check     // that the full file migration has been run.     $migration = $this->getMigration('d6_upload');
    $migration->set('requirements', []);
    $this->executeMigration($migration);
  }

  /** * Tests upload migration from Drupal 6 to Drupal 8. */