Necessary for web legacy compatibility.

Also fold in additional strict mode checks into LHS checks.
Minor constness clean-ups on the way.

R=marja@chromium.org
BUG=chromium:358346
LOG=Y

Review URL: https://codereview.chromium.org/217823003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20428 ce2b1a6d-e550-0410-aec6-3dcde31c8c00