Bitbucket does not support searching the source code of a repository.
It seems that source code search (and really, a repo-wide search that would cover code, wiki, issues, etc) would be extremely useful. Once repo-level searching is implemented, then maybe it could be extended to global Bitbucket search functionality across all available repos as well (similar to what Github allows, but hopefully without Github's lacking ease-of-use, as searching a specific repo at Github is definitely not intuitive).
EDIT: Please click to Vote for this issue (available to the right), rather than adding a +1. Only add a comment to this thread if you have something to say other than +1 or "me too". Otherwise you're just spamming the rest of us.
EDIT 2: Obviously this request isn't drawing enough attention from Bitbucket, as we have almost 1000 votes right now, 300+ comments, and no resolution or updates from Bitbucket. So, let's turn up the heat!
Post on Twitter, Facebook, Google+, and tag Bitbucket.
Tell them you want this feature. Use hashtag #bbynosearch