Earlier quoted context omitted.
It's an optional mode via command line, no? Opt-in won't require anyone to use closure.
Fair enough ... it's definitely worth considering seriously. I'd probably wait until it's "finished" according to Michael, and then look at if it's possible to add enough hooks to the compiler for it to be implemented as a plugin. Ideally you'd be able to emit different annotations depending on your use case, for example: lots of other folks have requested automatic JSDoc annotations.
As this would greatly help the closure compiler do optimisations, and since it is opt-in to add the typed parameter and would only produce a javascript comment above said variable.
For example....
foo /* @type {String} */ var foo = "hey"
I don't think this is deviating from CoffeeScript's aim to keep things pure? Is it?