We did not properly combine the property names from the parameter map
and the arguments backing store.  They could overwrite each other and
be unsorted.

Also fix an unrelated bug: deleting from a dictionary-mode arguments
backing store could corrupt the parameter map.

R=rossberg@chromium.org
BUG=1531
TEST=mjsunit/regress/regress-1531.js

Review URL: http://codereview.chromium.org/7278033

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8571 ce2b1a6d-e550-0410-aec6-3dcde31c8c00