Make StreamController be a StreamSink, not just an EventSink.

sdk/lib/async/stream_impl.dart

Index: sdk/lib/async/stream_impl.dart
diff --git a/sdk/lib/async/stream_impl.dart b/sdk/lib/async/stream_impl.dart
index 85aa0833b6f0233cf7399ffb256e21aee5261169..108e42b7d7dd50fa54f1e6d4a9f5b9e86ebd6f52 100644
--- a/sdk/lib/async/stream_impl.dart
+++ b/sdk/lib/async/stream_impl.dart
@@ -719,8 +719,11 @@ class _AsBroadcastStream<T> extends Stream<T> {
                                      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() {