Redo StreamTransformers so they work with Stack traces.

sdk/lib/async/async_sources.gypi

 # 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.
 
 # This file contains all sources for the dart:async library.
 {
   'sources': [
     'async.dart',
     # The above file needs to be first as it lists the parts below.
     'async_error.dart',
     'broadcast_stream_controller.dart',
     'deferred_load.dart',
     'event_loop.dart',
     'future.dart',
     'future_impl.dart',
     'stream.dart',
     'stream_controller.dart',
     'stream_impl.dart',
     'stream_pipe.dart',
+    'stream_transformers.dart',
     'timer.dart',
     'zone.dart',
   ],
 }