Switch to async_helper package.

tests/lib/convert/encoding_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 "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:convert';
 import 'unicode_tests.dart';
-import '../../async_helper.dart';
+import "package:async_helper/async_helper.dart";
 
 void runTest(List<int> bytes, expected) {
   var controller = new StreamController();
   asyncStart();
   UTF8.decodeStream(controller.stream).then((decoded) {
     Expect.equals(expected, decoded);
     asyncEnd();
   });
   int i = 0;
   while (i < bytes.length) {
@@ skipping 10 matching lines @@
 }
 
 
 main() {
   for (var test in UNICODE_TESTS) {
     var bytes = test[0];
     var expected = test[1];
     runTest(bytes, expected);
   }
 }