Port r14563 (6d01adb8)

Original commit message:
Generators now box their return values in object literals of the form

  { value: VAL, done: DONE }

where DONE is false for yield expressions, and true for return
statements.

BUG=

Review URL: https://codereview.chromium.org/15009008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14582 ce2b1a6d-e550-0410-aec6-3dcde31c8c00