9 files declare their use of FixerBase in Drupal 7 to 8/9 Module Upgrader 8

CreateClass.php in src/Plugin/DMU/Fixer/CreateClass.php

Define.php in src/Plugin/DMU/Fixer/Define.php

Delete.php in src/Plugin/DMU/Fixer/Delete.php

FormCallbackToMethod.php in src/Plugin/DMU/Fixer/FormCallbackToMethod.php

HookToYAML.php in src/Plugin/DMU/Fixer/HookToYAML.php

Implement.php in src/Plugin/DMU/Fixer/Implement.php

ImplementHook.php in src/Plugin/DMU/Fixer/ImplementHook.php

Notify.php in src/Plugin/DMU/Fixer/Notify.php

PSR4.php in src/Plugin/DMU/Fixer/PSR4.php