[flow] support arrows (short lambda)

Summary: Mostly a copy of what I've done for PHP 1 month ago.

Test Plan:
unit test included
www parse at 95% (was 85% before)

Reviewers: avik

Reviewed By: avik

CC: bryano

Differential Revision: https://phabricator.fb.com/D1207759
* lang_ml/parsing/ast_ml.ml: removed NotParsedCorrectly
* main_codemap.ml: comments
clean lang_js/ remove avik's code, remove NotParsedCorrectly, FinalDef
* changes.txt: