Large codebases in dynamic languages are the pits. Determining if some bit of code is used and how is a probabilistic exercise. Eventually you learn to give up or YOLO and see what happens.
If I write the tests, maybe. Some tests make refactoring even harder. If I couldn't already refactor my own code we're also in pretty bad trouble. If I've written more than 10% of the tests overall, things have already gone badly.
