Allow IfBuilder's to join existing (captured) continuations.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/23452049

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16857 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
remove remaining uses of default isolate in tests

R=svenpanne@chromium.org
BUG=

Review URL: https://codereview.chromium.org/23929006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16858 ce2b1a6d-e550-0410-aec6-3dcde31c8c00