Make synchronous broadcast StreamController throw if adding event while adding event.

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 c2cd4ab6c1c26f544961330d68b95ece2a24f41f..93c0833317c26f2393dd14fb46aea127ca1edf4b 100644
--- a/sdk/lib/async/broadcast_stream_controller.dart
+++ b/sdk/lib/async/broadcast_stream_controller.dart
@@ -337,12 +337,23 @@ abstract class _BroadcastStreamController<T>
   }
 }
 
-class _SyncBroadcastStreamController<T> extends _BroadcastStreamController<T> {
+class _SyncBroadcastStreamController<T> extends _BroadcastStreamController<T>
+                                     implements SynchronousStreamController<T> {
   _SyncBroadcastStreamController(void onListen(), void onCancel())
       : super(onListen, onCancel);
 
   // EventDispatch interface.
 
+  bool get _mayAddEvent => super._mayAddEvent && !_isFiring;
+
+  _addEventError() {
+    if (_isFiring) {
+      return new StateError(
+          "Cannot fire new event. Controller is already firing an event");
+    }
+    return super._addEventError();
+  }
+
   void _sendData(T data) {
     if (_isEmpty) return;
     if (_hasOneListener) {