Make Stream transformation respect the single/multi subscriber nature of the source.

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;
 
 // -------------------------------------------------------------------
 // Default implementation of a stream with a controller for adding
 // events to the stream.
 // -------------------------------------------------------------------
@@ skipping 38 matching lines @@
   /**
    * A controller with a [stream] that supports only one single subscriber.
    * The controller will buffer all incoming events until the subscriber is
    * registered.
    */
   StreamController.singleSubscription() {
     _stream = new _SingleControllerStream<T>(onSubscriptionStateChange,
                                              onPauseStateChange);
   }
 
+  bool get isSingleSubscription => _stream.isSingleSubscription;
+
   StreamSubscription listen(void onData(T data),
                             { void onError(AsyncError error),
                               void onDone(),
                               bool unsubscribeOnError}) {
     return _stream.listen(onData,
                           onError: onError,
                           onDone: onDone,
                           unsubscribeOnError: unsubscribeOnError);
   }
 
@@ skipping 94 matching lines @@
   _SingleControllerStream(this._subscriptionHandler, this._pauseHandler);
 
   void _onSubscriptionStateChange() {
     _subscriptionHandler();
   }
 
   void _onPauseStateChange() {
     _pauseHandler();
   }
 }