Update documentation for "close".

sdk/lib/async/broadcast_stream_controller.dart

Index: sdk/lib/async/broadcast_stream_controller.dart
diff --git a/sdk/lib/async/broadcast_stream_controller.dart b/sdk/lib/async/broadcast_stream_controller.dart
index 18ff6543f1067fb57f2cee52b341e4dbe20fc3da..5ec2756f1fbe3e93a8083bd6b5795c924f0e0b24 100644
--- a/sdk/lib/async/broadcast_stream_controller.dart
+++ b/sdk/lib/async/broadcast_stream_controller.dart
@@ -39,7 +39,6 @@ class _BroadcastSubscription<T> extends _ControllerSubscription<T>
   bool _expectsEvent(int eventId) =>
       (_eventState & _STATE_EVENT_ID) == eventId;
 
-
   void _toggleEventId() {
     _eventState ^= _STATE_EVENT_ID;
   }
@@ -181,7 +180,7 @@ abstract class _BroadcastStreamController<T>
 
   StreamSubscription<T> _subscribe(bool cancelOnError) {
     if (isClosed) {
-      throw new StateError("Subscribing to closed stream");
+      return new _DoneStreamSubscription<T>(_nullDoneHandler);
     }
     StreamSubscription subscription =
         new _BroadcastSubscription<T>(this, cancelOnError);