Use relative URIs for core library part-of statements.

sdk/lib/async/stream.dart

 // Copyright (c) 2013, 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 "async.dart";
 
 // -------------------------------------------------------------------
 // Core Stream types
 // -------------------------------------------------------------------
 
 typedef void _TimerCallback();
 
 /**
  * A source of asynchronous data events.
  *
@@ skipping 1974 matching lines @@
   }
 
   void addError(error, [StackTrace stackTrace]) {
     _sink.addError(error, stackTrace);
   }
 
   void close() {
     _sink.close();
   }
 }