Switch to async_helper package.

tests/compiler/dart2js_extra/deferred/deferred_constant_test.dart

 // Copyright (c) 2013, 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 '../../../async_helper.dart';
-
+import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 
 import 'dart:async';
 
 @lazy import 'deferred_class_library.dart';
 
 const lazy = const DeferredLibrary('deferred_class_library');
 
 main() {
   var x;
@@ skipping 23 matching lines @@
     Expect.equals(42, x.foo(87));
     Expect.listEquals(const [const Constant(42)], [new Constant(42)]);
     asyncEnd();
   });
   Expect.equals(0, counter);
   Expect.isNull(x);
   Expect.isNull(const Constant(42));
   Expect.isNull(const [const Constant(42)]);
   Expect.isNull(x);
 }