Fatal error: Uncaught Error: Class "Drupal\migrate\ProcessPluginBase" not found in /home/www/drupal8/test/core/modules/user/src/Plugin/migrate/process/UserLangcode.php:19 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/user/src/Plugin/migrate/process/UserLangcode.php on line 19