Unexplained async behaviour

my_test.dart

@TestOn('browser');
import 'package:test/test.dart';
import 'package:http/browser_client.dart';
import 'package:http_exception/http_status.dart';

main() {
  final http = new BrowserClient();
  test('Http request', () async {
      int start = new DateTime.now().millisecondsSinceEpoch;
      final response = await http.get('http://localhost');
      int duration = new DateTime.now().millisecondsSinceEpoch - start;
      int result = DateTime.parse(response.body).millisecondsSinceEpoch;
      expect(result, closeTo(start, duration);
  });
}