bug with for-of iteration of traceur/es6fiddle?

gistfile1.js

var obj = {
	_step: 0
};

obj[Symbol.iterator] = function() {
	return {
		next: function() {
			return { value: ++obj._step, done: (obj._step === 4) };
		}
	};
};

for (var v of obj){
  console.log(v);
}

// Expect:
//   1
//   2
//   3
//   4

// Get (via traceur and es6fiddle):
//   1
//   2
//   3