use Base in Watchman
use Base in Watchman

Summary: splits off some of hhugo's changes from https://github.com/facebook/flow/pull/8362 plus some additional changes to build without warnings.

Reviewed By: dsainati1

Differential Revision: D21620532

fbshipit-source-id: c99a677ec4f693cfb04a6e5e58894d873ec2c4df

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