This is one part of a WIP implementation of the stage-2 proposal to add
fields to classes: https://github.com/tc39/proposal-class-public-fields

See design doc:
https://docs.google.com/document/d/1WRtNm3ZLNJT1WVr8aq4RJuByYgfuAFAhj20LwTW6JVE/

This adds the desugaring logic to the parser. It isn't usable without
the (forthcoming) backend changes.

BUG=v8:5367

Review-Url: https://codereview.chromium.org/2316233004
Cr-Commit-Position: refs/heads/master@{#39460}