Enable "part of" for the async library.

sdk/lib/async/stream_controller.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.
 
-// part of dart.async;
+part of dart.async;
 
 // -------------------------------------------------------------------
 // Default implementation of a stream with a controller for adding
 // events to the stream.
 // -------------------------------------------------------------------
 
 /**
  * A controller and the stream it controls.
  *
  * This controller allows sending data, error and done events on
@@ skipping 131 matching lines @@
   _SingleControllerStream(this._subscriptionHandler, this._pauseHandler);
 
   void _onSubscriptionStateChange() {
     _subscriptionHandler();
   }
 
   void _onPauseStateChange() {
     _pauseHandler();
   }
 }