Fatal error: Uncaught Error: Class "Drupal\Tests\BrowserTestBase" not found in /home/www/drupal8/test/core/tests/Drupal/FunctionalTests/Routing/DefaultFormatTest.php:10
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/core/tests/Drupal/FunctionalTests/Routing/DefaultFormatTest.php on line 10