been missing for a long time but the fallthrough to the default case
used to break.  Now the default case should be unreachable and the
fallthrough therefore causes assertion failures.

Review URL: http://codereview.chromium.org/651024

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3913 ce2b1a6d-e550-0410-aec6-3dcde31c8c00