Check that environments assigned via AssignEnvironment are actually used.

Removed some temporary marker comments on the way.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/218403006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20430 ce2b1a6d-e550-0410-aec6-3dcde31c8c00