Enable "part of" for the async library.

sdk/lib/async/merge_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;
 
 class _SupercedeEntry<T> {
   final SupercedeStream stream;
   Stream<T> source;
   StreamSubscription subscription = null;
   _SupercedeEntry next;
 
   _SupercedeEntry(this.stream, this.source, this.next);
 
   // Whether the source stream is complete.
@@ skipping 257 matching lines @@
     if (_currentEntry.next == null) {
       _close();
       _currentEntry = _lastEntry = null;
     } else {
       // Remove the current entry from the list now that it's complete.
       _currentEntry = _currentEntry.next;
       _currentEntry.activate();
     }
   }
 }