How can I find an unused TCP port in Dart?

I'll add my 'hack' as an answer because I can't think of another way, and it seems to work pretty well.

Here's my getUnusedPort() function:

import 'dart:io';

Future<int> getUnusedPort(InternetAddress address) {
  return ServerSocket.bind(address ?? InternetAddress.anyIPv4, 0).then((socket) {
    var port = socket.port;
    socket.close();
    return port;
  });
}