Make asBroadcastStream take two callbacks.

sdk/lib/async/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;
 
 // -------------------------------------------------------------------
 // Controller for creating and adding events to a stream.
 // -------------------------------------------------------------------
 
@@ skipping 646 matching lines @@
     if (_isEmpty) return;
     _forEachListener((_BufferingStreamSubscription<T> subscription) {
       subscription._addError(error);
     });
   }
 
   void _sendDone() {
     if (_isEmpty) return;
     _forEachListener((_BroadcastSubscription<T> subscription) {
       subscription._close();
-      subscription._eventState |=
-          _BroadcastSubscription._STATE_REMOVE_AFTER_FIRING;
     });
   }
 }
 
 class _AsyncBroadcastStreamController<T> extends _BroadcastStreamController<T> {
   _AsyncBroadcastStreamController(void onListen(), void onCancel())
       : super(onListen, onCancel);
 
   // EventDispatch interface.
 
@@ skipping 86 matching lines @@
   }
 
   void _callOnCancel() {
     if (_hasPending) {
       _pending.clear();
       _pending = null;
     }
     super._callOnCancel();
   }
 }