Where is OAuth.registerService called?

I am trying to set up an Oauth2 server and clients. I am having trouble at a particular step, where there has been a popup, which then submits data to the endpoint :/_oauth/

The handler for this endpoint appears to be registered by a call to
OAuth.registerService

This function is defined at meteor/packages/oauth/oauth_server.js at 7da5b32d7882b510df8aa2002f891fc4e1ae1126 · meteor/meteor · GitHub

I would very much like to look at this handler, since my form submission seems to not be working. But where is it? If I could just see where OAuth.registerService is called, that would help a lot.

I have looked in several packages, such as accounts-base, accounts-oauth, accounts-google, accounts-github and accounts-lea with no success.

I could really use any kind of hint here.

Used in the respective oauth packages

e.g. github