I've never used it, but
https://github.com/istanbuljs/istanbuljs looks promising. I believe they should have sourcemap support in some form (e.g.
https://github.com/SitePen/remap-istanbul). From the looks of it you should be able to run Istanbul with the testing framework of your choice, and then remap the resulting coverage onto the original source code. If you could try this out and share your findings it would be awesome 😃