Defend against false positives

tests/language_strong/await_in_cascade_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'package:expect/expect.dart';
-import "package:async_helper/async_helper.dart";
 
 class C {
   Future<List<int>> m() async => []..add(await _m());
   Future<int> _m() async => 42;
 }
 
 main() async {
-  asyncStart();
   Expect.equals((await new C().m()).first, 42);
-  asyncEnd();
 }