code: function fireEvent(element, type, extra) {
    var event = doc.createEvent('HTMLEvents')
    event.initEvent(type, true, true)
  
    if (typeof extra === 'object') {
      for (var p in extra) {
        event[p] = extra[p]
      }
    }
  
    /**
     * A flag to distinguish with other events with the same name generated
     * by another library in the same page.
     */ 
    event._for = 'weex'
  
    element.dispatchEvent(event)
  }
docs: fire a HTMLEvent
  
  @param  {Element} element which element to fire a event on
  @param  {string}  type    type of event
  @param  {object}  extra   extra data for the event object