Switch to async_helper package.

tests/lib/convert/streamed_conversion_json_utf8_encode_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 'json_unicode_tests.dart';
-import '../../async_helper.dart';
+import "package:async_helper/async_helper.dart";
 
 final JSON_UTF8 = JSON.fuse(UTF8);
 
 
 Stream<List<int>> encode(Object o) {
   var controller;
   controller = new StreamController(onListen: () {
     controller.add(o);
     controller.close();
   });
@@ skipping 24 matching lines @@
 }
 
 void main() {
   for (var test in JSON_UNICODE_TESTS) {
     var expected = test[0];
     var object = test[1];
     testUnpaused(expected, encode(object));
     testWithPauses(expected, encode(object));
   }
 }