ES6: Implement generator method shorthand

https://people.mozilla.org/~jorendorff/es6-draft.html#sec-method-definitions

BUG=v8:3516
LOG=Y
R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/577973002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24048 ce2b1a6d-e550-0410-aec6-3dcde31c8c00