As required by the spec, and implemented by other browsers.

(Plus minor clean-up for redeclaration TypeErrors.)

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

Review URL: https://codereview.chromium.org/220473014

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20434 ce2b1a6d-e550-0410-aec6-3dcde31c8c00