The WeakMap constructor didn't have a unique prototype, so it shared one with
Object. All WeakMap functions (including "get" and "set") were installed on
that prototype.

R=rossberg@chromium.org
BUG=v8:1617
TEST=mjsunit/harmony/weakmaps

Review URL: http://codereview.chromium.org/7658008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8941 ce2b1a6d-e550-0410-aec6-3dcde31c8c00