It is an *unambiguous violation of ethics* for a collaborator to be dropped from the list of authors.

If you wrote a significant contribution to the paper, **you are in the right.** Your supervisor has no ground to stand on. You should *ask* your supervisor (don't accuse!) if he/she has included you as a co-author. If a mistake has been made, the sooner it's fixed the better. You could also have a good claim to first authorship, though that is something to decide between authors.