Resolve KeysTs in unions
Resolve KeysTs in unions

Summary: `KeysT`s can also hide potentially optimizable unions, so we wait to resolve them before proceeding.

Reviewed By: avikchaudhuri

Differential Revision: D18179553

fbshipit-source-id: 74236c364bce2483b2dd4602d2d39d114f1eb574

samwgoldman/flow,mroch/flow,samwgoldman/flow,facebook/flow,mroch/flow,mroch/flow,samwgoldman/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,samwgoldman/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,samwgoldman/flow,samwgoldman/flow,samwgoldman/flow,facebook/flow,mroch/flow