Switch to async_helper package.

tests/compiler/dart2js_extra/deferred/deferred_function_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.
 
 // Test that loading of a library (with top-level functions only) can
 // be deferred.
 
-import '../../../async_helper.dart';
-
+import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 
 import 'dart:async';
 
 @lazy import 'deferred_function_library.dart';
 
 const lazy = const DeferredLibrary('deferred_function_library');
 
 isNoSuchMethodError(e) => e is NoSuchMethodError;
 
@@ skipping 24 matching lines @@
     Expect.equals(2, ++counter);
     print('lazy was loaded');
     Expect.equals(42, foo('b'));
     Expect.isNotNull(readFoo());
     asyncEnd();
   });
   Expect.equals(0, counter);
   Expect.throws(() { foo('a'); }, isNoSuchMethodError);
   Expect.isNull(readFoo());
 }