Fatal error: Uncaught Error: Class "Drupal\Component\Serialization\Yaml" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Serialization/Yaml.php:13 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Serialization/Yaml.php on line 13