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