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

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

R=jkummerow@chromium.org, titzer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/102563004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18719 ce2b1a6d-e550-0410-aec6-3dcde31c8c00