Fatal error: Uncaught Error: Class "Drupal\Core\Form\FormBase" not found in /home/www/drupal8/test/core/modules/update/src/Form/UpdateManagerInstall.php:18
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/core/modules/update/src/Form/UpdateManagerInstall.php on line 18