Parse harmony let declarations.

Parse harmony let declarations.

Implementation of the harmony block scoped let bindings as proposed here:
http://wiki.ecmascript.org/doku.php?id=harmony:block_scoped_bindings

Changes to the syntax are explained there. They are active under the
harmony_block_scoping_ flag in the parser.

Review URL: http://codereview.chromium.org/7616009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8944 ce2b1a6d-e550-0410-aec6-3dcde31c8c00