As invocation of a functin may trigger GC and hence clear all the caches,
it's hard to ensure previous invariants.

Current invariant is both a key and a value should be the holes or not holes.

Review URL: http://codereview.chromium.org/6309004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6338 ce2b1a6d-e550-0410-aec6-3dcde31c8c00