Fatal error: Uncaught Error: Class "Drupal\Tests\BrowserTestBase" not found in /home/www/drupal8/test/core/modules/menu_ui/tests/src/Functional/MenuUiLanguageTest.php:18 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/menu_ui/tests/src/Functional/MenuUiLanguageTest.php on line 18