Add missing null-tests to async error functions.

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 cda7c0f960c7ce16effca089b555da257c8c7bc8..c2cd4ab6c1c26f544961330d68b95ece2a24f41f 100644
--- a/sdk/lib/async/broadcast_stream_controller.dart
+++ b/sdk/lib/async/broadcast_stream_controller.dart
@@ -238,10 +238,11 @@ abstract class _BroadcastStreamController<T>
   }
 
   void addError(Object error, [StackTrace stackTrace]) {
+    error = _nonNullError(error);
     if (!_mayAddEvent) throw _addEventError();
     AsyncError replacement = Zone.current.errorCallback(error, stackTrace);
     if (replacement != null) {
-      error = replacement.error;
+      error = _nonNullError(replacement.error);
       stackTrace = replacement.stackTrace;
     }
     _sendError(error, stackTrace);