Switch to async_helper package.

tests/lib/convert/streamed_conversion_utf8_decode_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<String> decode(List<int> bytes, int chunkSize) {
   var controller;
   controller = new StreamController(onListen: () {
     int i = 0;
     while (i < bytes.length) {
       List nextChunk = [];
       for (int j = 0; j < chunkSize; j++) {
         if (i < bytes.length) {
           nextChunk.add(bytes[i]);
@@ skipping 37 matching lines @@
     testUnpaused(expected, decode(bytes, 1));
     testWithPauses(expected, decode(bytes, 1));
     testUnpaused(expected, decode(bytes, 2));
     testWithPauses(expected, decode(bytes, 2));
     testUnpaused(expected, decode(bytes, 3));
     testWithPauses(expected, decode(bytes, 3));
     testUnpaused(expected, decode(bytes, 4));
     testWithPauses(expected, decode(bytes, 4));
   }
 }