Migrated test block 171 to Dart 2.0.

tests/lib_2/async/slow_consumer_test.dart

 // Copyright (c) 2012, 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.
 
 // VMOptions=--old_gen_heap_size=64
 
 library slow_consumer_test;
 
 import 'package:async_helper/async_helper.dart';
 import "package:expect/expect.dart";
@@ skipping 103 matching lines @@
   // heap-space.
 
   new DataProvider(800 * MB, 100 * MB, 1 * MB)
       .stream
       .pipe(new SlowConsumer(200 * MB))
       .then((count) {
     Expect.equals(100 * MB, count);
     asyncEnd();
   });
 }