Fatal error: Uncaught Error: Class "Drupal\Component\Plugin\Definition\PluginDefinition" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Layout/LayoutDefinition.php:14 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Layout/LayoutDefinition.php on line 14