_open() {
    if (this.transports.length === 0) {
      this.setTimeoutFn(() => {
        this.emitReserved("error", "No transports available");
      }, 0);
      return;
    }
    const transportName = this.opts.rememberUpgrade && _SocketWithoutUpgrade.priorWebsocketSuccess && this.transports.indexOf("websocket") !== -1 ? "websocket" : this.transports[0];
    this.readyState = "opening";
    const transport = this.createTransport(transportName);
    transport.open();
    this.setTransport(transport);
  }
Initializes transport to use and starts probe.

@private