Well this is a slightly embarrassing oversight, thanks for catching it! (I should have paid more attention to https://github.com/mozilla-b2g/gaia/blob/master/shared/js/async_storage.js)

I'll follow-up with a PR for https://github.com/mozilla/botio-files-pdfjs/blob/master/b2g_stubs/js/async_storage.js, so that we don't break the B2G preview.