You seem more uptight about it than I am. I didn't see it as anything so formal as "plagiarism", I just thought it was rude and ungrateful. If digging up work from 2014 is easy, you must be more organised than I am.
If you made a attempt to upstream the work (what most people would assume from “contributed”), it will be in Gerrit [1], which allows it to be found using the “owner:<email>” search.
[1]: go-review.googlesource.com