You are right. I also want it.
But in current version is Atom, prefixes started with special characters(such like `.` or `$`) aren't work. So if I add form of `.toJson` instead `toJson`, user can't autocomplete it after `angular.`.
It seems Atom's bug. Therefore I post the issue on atom community. It's particularly problem for us because angularjs has a lot of properties or function started with `$`.