Switch to async_helper package.

tests/light_unittest.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.
 
 /// A light-weight replacement for package:unittest.  This library runs tests
 /// synchronously, and avoids using reflection.
 library light_unittest;
 
 import 'dart:async';
 
-import 'async_helper.dart';
+import 'package:async_helper/async_helper.dart';
 import '../pkg/expect/lib/expect.dart';
 
 test(name, f) {
   print('Testing $name');
   try {
     f();
     print('PASS: $name');
   } catch (e, trace) {
     print('FAIL: $name.');
     print(e);
@@ skipping 37 matching lines @@
 expectAsync1(then) {
   asyncStart();
   return (x) {
     // 'then(x)' may call 'asyncStart()', so we first need to execute it, before
     // we can call 'asyncEnd()'.
     var result = then(x);
     asyncEnd();
     return result;
   };
 }