Port r18719 (13303b7a)

Original commit message:
Clean up some zone/isolate handling in AST and its visitors on the way.

(Based on https://codereview.chromium.org/103743004/)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/143873005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18726 ce2b1a6d-e550-0410-aec6-3dcde31c8c00