Fatal error: Uncaught Error: Class "Drupal\Tests\BrowserTestBase" not found in /home/www/drupal8/test/core/modules/config/tests/src/Functional/ConfigExportImportUITest.php:19
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/core/modules/config/tests/src/Functional/ConfigExportImportUITest.php on line 19