Fatal error: Uncaught Error: Class "Drupal\Core\Config\Development\ConfigSchemaChecker" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Config/Testing/ConfigSchemaChecker.php:21 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Config/Testing/ConfigSchemaChecker.php on line 21