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

sdk/lib/async/broadcast_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;
 
 class _BroadcastStream<T> extends _ControllerStream<T> {
   _BroadcastStream(_StreamControllerLifecycle controller) : super(controller);
 
   bool get isBroadcast => true;
@@ skipping 319 matching lines @@
   void _callOnCancel() {
     assert(_isEmpty);
     if (isClosed && _doneFuture._mayComplete) {
       // When closed, _doneFuture is not null.
       _doneFuture._asyncComplete(null);
     }
     _runGuarded(_onCancel);
   }
 }
 
-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) {
       _state |= _BroadcastStreamController._STATE_FIRING;
       _BroadcastSubscription subscription = _next;
       subscription._add(data);
       _state &= ~_BroadcastStreamController._STATE_FIRING;
       if (_isEmpty) {
         _callOnCancel();
       }
@@ skipping 147 matching lines @@
     _pauseCount++;
   }
   void resume() { _resume(null); }
   void _resume(_) {
     if (_pauseCount > 0) _pauseCount--;
   }
   Future cancel() { return new _Future.immediate(null); }
   bool get isPaused => _pauseCount > 0;
   Future asFuture([Object value]) => new _Future();
 }