ES6 symbols: prevent reflection, proxy, and observe APIs from leaking symbols

R=svenpanne@chromium.org
BUG=v8:2158

Review URL: https://codereview.chromium.org/12422019

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14056 ce2b1a6d-e550-0410-aec6-3dcde31c8c00