Fixed d8 shutdown.

It is crucial that the Isolate is cleanly disposed, otherwise the
optimizing compiler thread happily continues to run.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/274093002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21214 ce2b1a6d-e550-0410-aec6-3dcde31c8c00