d8 shell fix for r14530

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/14907004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14539 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Pretenure ASCII cons string in high promotion mode.

BUG=

Review URL: https://codereview.chromium.org/14451003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14540 ce2b1a6d-e550-0410-aec6-3dcde31c8c00