Enable "part of" for the async library.

sdk/lib/async/stream.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;
 
 // -------------------------------------------------------------------
 // Core Stream types
 // -------------------------------------------------------------------
 
 abstract class Stream<T> {
   Stream();
 
   factory Stream.fromFuture(Future<T> future) {
     var controller = new StreamController<T>();
@@ skipping 817 matching lines @@
     sink.signalError(error);
   }
 
   /**
    * Handle an incoming done event.
    */
   void handleDone(StreamSink<T> sink) {
     sink.close();
   }
 }