Use asserts instead of branches to avoid fast array methods for observed arrays

Because observed arrays should always be in dictionary mode, we'll always fall
back to the JS version anyway.

Review URL: https://codereview.chromium.org/12221033

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13612 ce2b1a6d-e550-0410-aec6-3dcde31c8c00