The js stuff in rP uses Javeline, which is mostly included in webroot/rsrc/externals/javelin/, but its support stuff (javelinsymbols linter) isn't.
It appears to live at https://github.com/phacility/javelin, and is a bit of a challenge to build (the instructions at https://secure.phabricator.com/book/javelin/article/facebook/ say "you must specify CXX explicitly because the default CXX is broken").
At a minimum, we should provide better instructions on how to build it, and maybe offer some binaries. At best, we should copy the whole support stuff and have it auto-build like xphast.
See also https://secure.phabricator.com/T2378 (which is still open, closing in on the 10 years mark)