Switch to async_helper package.

tests/lib/convert/streamed_conversion_json_encode1_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 '../../async_helper.dart';
+import "package:async_helper/async_helper.dart";
 
 final TESTS = [
     [ 5, '5' ],
     [ -42, '-42' ],
     [ 3.14, '3.14' ],
     [ true, 'true' ],
     [ false, 'false' ],
     [ null, 'null' ],
     [ 'quote"or\'', '"quote\\"or\'"' ],
     [ '', '""' ],
@@ skipping 49 matching lines @@
 }
 
 void main() {
   for (var test in TESTS) {
     var o = test[0];
     var expected = test[1];
     testNoPause(expected, o);
     testWithPause(expected, o);
   }
 }