Add missing null-tests to async error functions.

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 220 matching lines @@
     assert(_isAddingStream);
     return new StateError("Cannot add new events while doing an addStream");
   }
 
   void add(T data) {
     if (!_mayAddEvent) throw _addEventError();
     _sendData(data);
   }
 
   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);
   }
 
   Future close() {
     if (isClosed) {
       assert(_doneFuture != null);
       return _doneFuture;
     }
@@ skipping 247 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();
 }