Fix setting of StopIteration and GeneratorReturn.

- Set the global StopIteration correctly in the 'undefined' case.
- Use getters so that the effects of setStopIteration, etc, show up in
  traceur.runtime.StopIteration, etc. Previously it was only updated
  during runtime initialization.

BUG=None
TEST=test/feature/Yield/StopIteration.js

Review URL: https://codereview.appspot.com/7757044
Patch from Ben Chan <usrbincc@yahoo.com>.