Fatal error: Uncaught Error: Class "Drupal\migrate_drupal\Plugin\migrate\field\FieldPluginBase" not found in /home/www/drupal8/test/core/modules/migrate_drupal/src/Plugin/migrate/field/UserReference.php:18
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/core/modules/migrate_drupal/src/Plugin/migrate/field/UserReference.php on line 18