Fatal error: Uncaught Error: Class "Drupal\Tests\BrowserTestBase" not found in /home/www/drupal8/test/core/modules/system/tests/src/Functional/System/TokenReplaceWebTest.php:14 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/system/tests/src/Functional/System/TokenReplaceWebTest.php on line 14