Fatal error: Uncaught Error: Class "Drupal\KernelTests\KernelTestBase" not found in /home/www/drupal8/test/core/tests/Drupal/KernelTests/Core/Theme/StableThemeTest.php:12 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/tests/Drupal/KernelTests/Core/Theme/StableThemeTest.php on line 12