Server not terminated when the program is stopped

Question

I made a simple server app with Dart that receives and returns messages. It works great except when I stop the program the server keeps running in the background and I have to go to the cmd to kill the task before running the program again.

Why does the server not terminate when the program is stopped, and how do I fix that?

import 'dart:io' show HttpServer, HttpRequest, WebSocket, WebSocketTransformer;

void main() {
  var sockets = <WebSocket>[];
  var i = 0;

  HttpServer.bind('localhost', 8080).then((HttpServer server) {
    print('[+]WebSocket listening at -- ws://localhost:8080/');
    server.listen((HttpRequest request) {
      WebSocketTransformer.upgrade(request).then((WebSocket ws) {
        sockets.add(ws);
        ws.listen(
          (data) {
            print(data);
            for(var socket in sockets) {
              socket.add('a msg back from the server: $i');
            }
            i++;
          },
          onDone: () => print('[+]Done :)'),
          onError: (err) => print('[!]Error -- ${err.toString()}'),
          cancelOnError: true,
        );
      }, onError: (err) => print('[!]Error -- ${err.toString()}'));
    }, onError: (err) => print('[!]Error -- ${err.toString()}'));
  }, onError: (err) => print('[!]Error -- ${err.toString()}'));
}

Thanks in advance for the help.