I can think of a couple ways to fix this
- Always return all errors, then have `tsc` have a filter when displaying errors for JS files. This would add errors to all other API consumers, though.
- Give the language service a separate way to request unchecked JS errors. The checker will have to store these errors separately and the language service will have to add them late.
- Possibly: Not report these errors in the "errors in closed files" list. But I don't know if that would be what users of that feature expect.

Opinions? Ideas?

I'm pretty sure that this feature is going to stick around and grow, so I'd like for it to be easier to use than to not use. Specifically, it's annoying if the future status quo for the language service is to request diagnostics twice, once for unchecked JS and again for everything else.