Handle special characters in JSON eval()

Summary: See https://medium.com/joys-of-javascript/42a28471221d.

Test Plan:
  lang=js
  eval('({"str": "own\u2028ed"})');
  eval('({"str": "own\u2028ed"})'.replace(/\u2028/g, '\\u2028').replace(/\u2029/g, '\\u2029'));

Reviewers: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4385