constructor (type, eventInitDict = {}) {
    type = webidl.converters.DOMString(type)
    eventInitDict = webidl.converters.ProgressEventInit(eventInitDict ?? {})

    super(type, eventInitDict)

    this[kState] = {
      lengthComputable: eventInitDict.lengthComputable,
      loaded: eventInitDict.loaded,
      total: eventInitDict.total
    }
  }
@see https://xhr.spec.whatwg.org/#progressevent