Fatal error: Uncaught Error: Class "Drupal\Tests\BrowserTestBase" not found in /home/www/drupal8/test/core/modules/system/tests/src/Functional/Theme/ThemeEarlyInitializationTest.php:13
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/core/modules/system/tests/src/Functional/Theme/ThemeEarlyInitializationTest.php on line 13