Enable "part of" for the async library.

sdk/lib/async/stream_impl.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;
 
 // States shared by single/multi stream implementations.
 
 /// Initial and default state where the stream can receive and send events.
 const int _STREAM_OPEN = 0;
 /// The stream has received a request to complete, but hasn't done so yet.
 /// No further events can be aded to the stream.
 const int _STREAM_CLOSED = 1;
 /// The stream has completed and will no longer receive or send events.
 /// Also counts as closed. The stream must not be paused when it's completed.
@@ skipping 1003 matching lines @@
     if (_isComplete) {
       throw new StateError("Subscription has been canceled.");
     }
     if (_timer != null) {
       _timer.cancel();
       _timer = null;
     }
     _pauseCount = 0;
   }
 }