Switch to async_helper package.

tests/lib/mirrors/invoke_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.
 
 library test.invoke_test;
 
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
-import '../../async_helper.dart';
+import "package:async_helper/async_helper.dart";
 
 class C {
   var field;
   C() : this.field = 'default';
   C.named(this.field);
 
   get getter => 'get $field';
   set setter(v) => field = 'set $v';
   method(x, y, z) => '$x+$y+$z';
   toString() => 'a C';
@@ skipping 325 matching lines @@
     Expect.equals(c, libraryField);
   });
   future = lm.setFieldAsync(const Symbol('doesntExist'), 'lbar');
   expectError(future, isNoSuchMethodError, 'Not defined');
 }
 
 main() {
   testSync();
   testAsync();
 }