Keep track of 'disallowIn' as an ambient parser flag instead of having to pass it along everywhere.

This is the style i would like to use for 'YieldExpressions' as it will save an enormous amount of effort trying to pass along the 'allowYield' flag everywhere. 
I have to read this, think about it for like 15 seconds, and eventually default to "Okay, what does the OTHER branch do?" Yes, my brain has a stack depth of 2, I can't do double negatives.

Just make it `if (disallowIn) {` and perform the other branch. It's the more "interesting" condition anyhow.