Fatal error: Uncaught Error: Interface "Drupal\Core\Render\Placeholder\PlaceholderStrategyInterface" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Render/Placeholder/SingleFlushStrategy.php:12 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Render/Placeholder/SingleFlushStrategy.php on line 12