Of note, the new bootstrap only adds a prefs URI to `sdk/addon/runner` if it exists -- `cfx` created a blank file for this which was always fetched with the addon runner. [A check to see](https://github.com/mozilla/addon-sdk/pull/1291/files#diff-6ad46ae895d0e667000f190d904a8725R143) if the URI exists was added to the runner, but maybe this can be a key of some sort.