watch.dart

import 'dart:async';
import 'dart:convert';
import 'dart:io';
import 'package:async/async.dart';
import 'package:watcher/watcher.dart';

watchCurrentDirectory(List<String> arguments) async {
  var watcher = new DirectoryWatcher(Directory.current.path);

  while (true) {
    print("Starting webserver...");

    var process = await Process.start('dart', arguments);

    await for (var event in StreamGroup.merge([watcher.events, process.stdout])) {
      if (event is WatchEvent) {
        print("Restarting...");

        process.kill();

        break;
      }
      else if (event is List<int>) {
        print(UTF8.decode(event));
      }
    }
  }
}

Future startServer() async {
  var counter = 0;
  var server = await HttpServer.bind(
    InternetAddress.LOOPBACK_IP_V4,
    4040);

  print('listening on localhost:${server.port}');

  server.listen((HttpRequest request) {
    print("Counter: $counter");

    request.response
      ..write('Test, world! $counter')
      ..close();

    counter++;
  });
}

main(List<String> arguments) async {
  if (arguments.length > 0 && arguments[0] == "watch")
  {
    var watchArguments = arguments.sublist(1)
                                  ..insert(0, Platform.script.toFilePath());

    watchCurrentDirectory(watchArguments);
  }
  else
  {
    await startServer();
  }
}

pubspec.yaml:

name: my_app
dependencies:
  watcher: ^0.9.7