"use strict" isn't comparable to warning flags at all, because it changes the behavior of code. I only brought it up because it can be applied selectively within function scope, not specifically for what it does.

I think what we need is a way to selectively disable language features. Changing the behavior of existing features is a much more questionable thing to do in my view.

The big question is if it's possible to disable some features without affecting how other features must work. It failed miserably for exceptions, but exceptions are a cross cutting sort of concern.