Fatal error: Uncaught Error: Class "Drupal\Core\Plugin\DefaultPluginManager" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Mail/MailManager.php:28 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Mail/MailManager.php on line 28