POC, simple transformation :)

gistfile1.js

function * foo() {
  a();
  yield x;
  b();
}

// =>

function * foo() {
  var started = false;
  var finished = false;
  
  function f1() {
    a();
  }
  
  function f2() {
    x;
    b();
  }
  
  return {
    next: function () {
      if (!started) {
        started = true;
        f1();
      } else if (!finished) {
        finished = true;
        f2();
      } else {
        throw 'nope';
      }
    };
  }
}