Fatal error: Uncaught Error: Class "Symfony\Component\EventDispatcher\GenericEvent" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Field/FieldStorageDefinitionEvent.php:10 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Field/FieldStorageDefinitionEvent.php on line 10