We've introduced Guice so we can test functionality using mocks. Making ReturnUrlVerifier a trait doesn't have the same effect for testability.

Testing the backend logic is likely less of an issue in a lot of the components in frontend so far. For Identity, we'll be doing a lot of work in the backend and having detailed and reliable tests is important to delivering a stable and secure Identity platform.

I know this is a contentious point so I'll refer you to @frj to argue about it in more detail.