That depends on how you install GHCJS. If you do a release build, starting from a ghcjs-x.y.z.tar.gz sdist (there are some prereleases out there to be used with stack), then all libraries are taken from this archive.
If you do a development build (boot with --dev), then the ghcjs-boot program clones the ghcjs-boot repository, which contains ghcjs-base as a submodule. The submodule determines exactly which ghcjs-base version you get.
I've been working on the ghcjs-base library recently, but since I had no way to automatically test things like WebSocket and XMLHttpRequest, much of that code has only seen rather spotty testing. I've now updated the GHCJS testsuite to support automated browser testing through Selenium, so I'll be adding tests and fixing things soon, and bump the submodule when I feel reasonably confident that it works.
However, unlike ghcjs-prim, ghcjs-base is a fairly normal package that you can install in your user package database or in a sandbox (there are still lots of low-level implementation details in there, so don't expect long term compiler compatibility, but bugfix updates should be ok). So just clone a copy and install it if you want to help test (or fix) the changes.
luite