Make `let` usable as an identifier in ES6 sloppy mode.

All of our mjsunit suite now runs through with --harmony-scoping enabled, up to expected failures (tests checking syntax errors for const/function in strict mode).

R=marja@chromium.org, ulan@chromium.org
BUG=v8:2198
LOG=Y

Review URL: https://codereview.chromium.org/378303003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22323 ce2b1a6d-e550-0410-aec6-3dcde31c8c00