Add isStopIteration to '@iter' module. Update the tests to use it.

BUG=None
TEST=test/feature/Yield/GeneratorClose.js
     test/feature/Yield/GeneratorSend.js
     test/feature/Yield/GeneratorReturn.js
     test/feature/Yield/GeneratorThrow.js

Review URL: https://codereview.appspot.com/7610045
Patch from Ben Chan <usrbincc@yahoo.com>.