This commit adds initial parser support for harmony iteration.
Specifically, it will parse:

  for (x of y) {}
  for (let x of y) {}
  for (var x of y) {}

The semantics are still unimplemented.

TEST=mjsunit/harmony/for-of-syntax
BUG=v8:2214
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/15300018

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14984 ce2b1a6d-e550-0410-aec6-3dcde31c8c00