Adjust test262-es6 following --harmony flag change

TBR=adamk@chromium.org
BUG=

Review URL: https://codereview.chromium.org/702753002

Cr-Commit-Position: refs/heads/master@{#25119}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25119 ce2b1a6d-e550-0410-aec6-3dcde31c8c00