Fatal error: Uncaught Error: Class "Drupal\Core\Plugin\DefaultPluginManager" not found in /home/www/drupal8/test/core/modules/aggregator/src/Plugin/AggregatorPluginManager.php:21 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/aggregator/src/Plugin/AggregatorPluginManager.php on line 21