Switch to async_helper package.

tests/lib/convert/streamed_conversion_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 'unicode_tests.dart';
-import '../../async_helper.dart';
+import "package:async_helper/async_helper.dart";
 
 Stream<List<int>> encode(String string, int chunkSize) {
   var controller;
   controller = new StreamController(onListen: () {
     int i = 0;
     while (i < string.length) {
       if (i + chunkSize <= string.length) {
         controller.add(string.substring(i, i + chunkSize));
       } else {
         controller.add(string.substring(i));
@@ skipping 32 matching lines @@
 main() {
   for (var test in UNICODE_TESTS) {
     var expected = test[0];
     var string = test[1];
     testUnpaused(expected, encode(string, 1));
     testWithPauses(expected, encode(string, 1));
     testUnpaused(expected, encode(string, 2));
     testWithPauses(expected, encode(string, 2));
   }
 }