Prevent non-js merge conflicts in Brocfile.js.

Prior to this, a file named `.DS_Store` in both the lib and test
directories of a single package would cause an error during the build.

This doesn't completely fix the issue, but does prevent the majority of
cases that are benign. If we end up having further merge conflicts where
we have a file with the same name in both `lib/` and `test/` dirs we can
address those individually.

This fixes the majority of trivial scenarios for now.
Merge pull request #4910 from rjackson/only-operate-on-js-files

Prevent non-js merge conflicts in Brocfile.js.