Microsoft/TypeScript — pull request #1123 by CyrusNajmabadi (2014-11-11 03:08:52+00:00)
Title: 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.