Fatal error: Uncaught Error: Class "Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldConstraint" not found in /home/www/drupal8/test/core/modules/user/src/Plugin/Validation/Constraint/UserNameUnique.php:15 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/user/src/Plugin/Validation/Constraint/UserNameUnique.php on line 15