Is there a geolocation API part of google closure (ultimately for use in ClojureScript) ?
I've been looking for it, but all I can find is something that looks like an old shim.
If there is none, is it because Google doesn't use that feature of web browsers (they find our location using other means), or because the browser API doesn't require that (already unified), and what would be the best alternative in ClojureScript ?
Nothing is in Closure for this right now. Generally, Closure only wraps these native APIs when there are cross-browser compatibility issues, is potential for a polyfill, or potential for improvement. I'm not personally too familiar with these APIs, but my guess is it's not in Closure because it's pretty standardized across the browsers that implement it.
The "shim" link you provided is what's called an externs file. It's used in conjunction with the Closure Compiler to avoid renaming certain properties. See this resource for more info.