With a local build on my buildbot of the tip of the default branch, pkgutil.get_importer('') returns FileFilter('.').  The tests also passed.

However, after checking here and realizing the offending code had been removed from the test, I put it back in and it still fails.

I think the problem is that sys.path[0] is not '' when testing in the buildbot environment (though it is when I run the debug interpreter interactively).  Within the buildbot scripts it's a path to the stdlib test zip, so for example in my test tree it's "D:\cygwin\home\db3l\test\build\PCbuild\python33_d.zip".

And if I hand that string to pkgutil.get_importer() I do seem to get None back.