In general I discourage use of loader.sandbox which we have added only in order to avoid rewriting old tests (which we'll do at some point in a future). So if there is any way to test API without an access to a sandbox that's much more preferred alternative. In this case I believe it should be possible to do it without emit by just changing an actual pref no ? If so do you mind doing it this way ?
We're almost there, sorry for not being very clear first time around. Also could you please merge with upstream/master cause otherwise it won't auto merge. (P.S. getting rid of changes in simple-prefs.js before merge would probably make it easier)