Revert "Change some platform libraries to use `part of URI`."

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 dart.async;
 
 // -------------------------------------------------------------------
 // Core Stream types
 // -------------------------------------------------------------------
 
 typedef void _TimerCallback();
 
 /**
  * A source of asynchronous data events.
  *
@@ skipping 1916 matching lines @@
   /// Must only be used instead of listening to the [values] stream.
   /// If the stream has been listened to, this call fails.
   /// After calling this method, listening on the [values] stream fails.
   Future cancel() {
     // If values has been listened to,
     // this throws a StateError saying that stream has already been listened to,
     // which is a correct error message for this call too.
     return values.listen(null).cancel();
   }
 }