Summary:
This supersedes #760.
Additional changes over #760:
Pinning Prettier version to get some formatting always. (Different prettier versions were responsible for the observed flip-flopping of StringPrototype.js.)
Print output of prettier in wrapper script for better diagnosis / experience. (TODO: How to just pipe through console output? Couldn't get it to work.)
Updating problematic .js file for the last time.
Closes https://github.com/facebook/prepack/pull/787
Differential Revision: D5382336
Pulled By: NTillmann
fbshipit-source-id: 95ed988ef3f091493d2fb509b5481753d56901cd
