The instance fields proposal is at stage 3, with implementation activity in other engines. While I'd expect some additional spec churn, we should implement this with high priority to suss out any potential performance concerns.

I mean the instance methods and accessors proposal, of course.

Pushed by avandolder@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/d1a5046cd636 Add pref for private methods. r=jorendorff

Pushed by avandolder@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/22e3020fa080 Implement private instance methods and accessors. r=jorendorff