Eliminates substantial amounts of fragile code duplication and special casing.

Also fixes "a".propertyIsEnumerable(0) to correctly return true.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/11420011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12990 ce2b1a6d-e550-0410-aec6-3dcde31c8c00