Fatal error: Uncaught Error: Class "Symfony\Component\Validator\Constraint" not found in /home/www/drupal8/test/core/modules/book/src/Plugin/Validation/Constraint/BookOutlineConstraint.php:15 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/book/src/Plugin/Validation/Constraint/BookOutlineConstraint.php on line 15