Make StreamController be a StreamSink, not just an EventSink.

sdk/lib/async/stream_impl.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;
 
 /** Abstract and private interface for a place to put events. */
 abstract class _EventSink<T> {
   void _add(T data);
   void _addError(Object error);
@@ skipping 701 matching lines @@
                                  void onDone(),
                                  bool cancelOnError}) {
     if (_controller == null) {
       throw new StateError("Source stream has been closed.");
     }
     if (_subscription == null) {
       _subscription = _source.listen(_controller.add,
                                      onError: _controller.addError,
                                      onDone: _controller.close);
     }
-    return _controller.stream.listen(onData, onError: onError, onDone: onDone,
-                                     cancelOnError: cancelOnError);
+    if (onData == null) onData = _nullDataHandler;
+    if (onError == null) onError = _nullErrorHandler;
+    if (onDone == null) onDone = _nullDoneHandler;
+    cancelOnError = identical(true, cancelOnError);
+    return _controller._subscribe(onData, onError, onDone, cancelOnError);
   }
 
   void _onCancel() {
     // Called by [_controller] when it has no subscribers left.
     StreamSubscription subscription = _subscription;
     _subscription = null;
     _controller = null;  // Marks the stream as no longer listenable.
     subscription.cancel();
   }
 }
@@ skipping 141 matching lines @@
       _FutureImpl<bool> hasNext = _futureOrPrefetch;
       _clear();
       hasNext._setValue(false);
       return;
     }
     _subscription.pause();
     _futureOrPrefetch = null;
     _state = _STATE_EXTRA_DONE;
   }
 }