Currently Transformer::new takes a source_type param:

Can we get rid of it? source_type is present in Program and parser updates it from "unambiguous" to either "script" or "module".

It seems to make more sense to use the value of source_type present in Program, than allow user to over-ride it.

I can't see any circumstance in which that's a useful thing to do, and it's a potential footgun. It's confusing that in transformer it's possible that self.ctx.source_type.is_script() != !self.ctx.source_type.is_module(), because it can also be "unambiguous". That's not theoretical - I hit this problem while trying to pass some TypeScript tests.

If for some obscure reason, the user does want to override it, they can always alter the value of Program::source_type before passing Program to transformer.