Fix sloppy-mode 'const' under Harmony flag.

R=yangguo@chromium.org
BUG=173361

Review URL: https://codereview.chromium.org/19199002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15670 ce2b1a6d-e550-0410-aec6-3dcde31c8c00