code: function attach(el, fn){
    el.attachEvent('onreadystatechange', function(e){
      if (!/complete|loaded/.test(el.readyState)) return;
      fn(null, e);
    });
    el.attachEvent('onerror', function(e){
      var err = new Error('failed to load the script "' + el.src + '"');
      err.event = e || window.event;
      fn(err);
    });
  }
docs: Attach event.
  
  @param {Element} el
  @param {Function} fn
  @api private