gistfile1.js

function spawn(generatorFunc) {
  function continuer(verb, arg) {
    var result;
    try {
      result = generator[verb](arg);
    } catch (err) {
      return Promise.reject(err);
    }
    if (result.done) {
      return result.value;
    } else {
      return Promise.resolve(result.value).then(onFulfilled, onRejected);
    }
  }
  var generator = generatorFunc();
  var onFulfilled = continuer.bind(continuer, "next");
  var onRejected = continuer.bind(continuer, "throw");
  return onFulfilled();
}

Thank you @jakearchibald for the showModalDialog replacement. It's really helpful. Glad I was able to find this simple solution.

@jakearchibald I have this. Although it doesn't handle returning a value, it has some advantages. :-)

function spawn(gen) {
	return function(...args) {
		let iter = gen(...args);
		function step({value, done}) {
			if (!done) value.then(v => iter.next(v), e => iter.throw(e)).then(step);
		}
		step(iter.next());
	}
}