Fatal error: Uncaught Error: Class "Drupal\migrate\ProcessPluginBase" not found in /home/www/drupal8/test/core/modules/file/src/Plugin/migrate/process/d6/FieldFile.php:18 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/file/src/Plugin/migrate/process/d6/FieldFile.php on line 18