Fatal error: Uncaught Error: Class "Drupal\Tests\BrowserTestBase" not found in /home/www/drupal8/test/core/modules/block/tests/src/Functional/BlockHookOperationTest.php:12
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/core/modules/block/tests/src/Functional/BlockHookOperationTest.php on line 12