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

sdk/lib/async/broadcast_stream_controller.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 _BroadcastStream<T> extends _ControllerStream<T> {
   _BroadcastStream(_StreamControllerLifecycle<T> controller)
       : super(controller);
 
   bool get isBroadcast => true;
 }
 
 class _BroadcastSubscription<T> extends _ControllerSubscription<T> {
   static const int _STATE_EVENT_ID = 1;
@@ skipping 491 matching lines @@
   }
 
   void _callOnCancel() {
     if (_hasPending) {
       _pending.clear();
       _pending = null;
     }
     super._callOnCancel();
   }
 }