flutter_tester process not shutting down

main.dart

import 'dart:async';
import 'dart:io';

Future<void> main() async {
  final testApp = TestFlutterApp(
    // For reproducing, this should be the path to a flutter app, relative to the directory
    // that this main.dart script lives in.
    'develop/devtools/packages/devtools_app/test/test_infra/fixtures/flutter_app',
  );
  await testApp.startProcess();
}

class TestFlutterApp {
  TestFlutterApp(this.appPath);

  final String appPath;

  Future<void> startProcess() async {
    final runProcess = await Process.start(
      'flutter',
      [
        'run',
        '--machine',
        '-d',
        'flutter-tester',
      ],
      workingDirectory: appPath,
    );
    final processId = runProcess.pid;

    print('waiting 20 sec for app to spin up');
    await Future.delayed(const Duration(seconds: 30));
    print('killing');

    // Tried both of these ways to kill the process.
    Process.killPid(processId, ProcessSignal.sigkill);
    // runProcess.kill(ProcessSignal.sigkill);
    print('after killing, waiting 10 sec');
    await Future.delayed(const Duration(seconds: 10));
  }
}