var channel = new MessageChannel();
  // linked list of tasks (single, with head node)
  var head = {}, tail = head;
  channel.port1.onmessage = function () {
      head = head.next;
      var task = head.task;
      delete head.task;
      task();
  };
  nextTick = function (task) {
      tail = tail.next = {task: task};
      channel.port2.postMessage(0);
  };