Spawn 10 dart2js: `out/ReleaseX64/dart dart2jsx10.dart --libraries-spec=sdk/lib/libraries.json hello.dart`

dart2jsx10.dart

import 'dart:async';
import 'dart:isolate';

import 'pkg/compiler/lib/src/dart2js.dart' as dart2jsMain;

void run(List<dynamic> params) async {
  final SendPort sendPort = params[0];
  final List arguments = params[1];
  await dart2jsMain.internalMain(arguments);
  sendPort.send(true);
}

void main(List<String> arguments) async {
  List<RawReceivePort> receivePorts = <RawReceivePort>[];
  List<Completer> completers = <Completer>[];
  for (int i = 0; i < 10; i++) {
    Completer completer = Completer();
    RawReceivePort receivePort = RawReceivePort((_) {
      completer.complete();
    });
    receivePorts.add(receivePort);
    completers.add(completer);
    Isolate.spawn(run, <dynamic>[receivePort.sendPort, arguments]);
  }

  await Future.wait(completers.map<Future>((completer) => completer.future));
  for (RawReceivePort receivePort in receivePorts) {
    receivePort.close();
  }
}