Fix warnings in library-tests.

tests/lib/async/event_helper.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.
 
 library event_helper;
 
 import 'dart:async';
 
 abstract class Event {
   void replay(EventSink sink);
@@ skipping 140 matching lines @@
 
   CaptureEvents(Stream stream,
                 { bool cancelOnError: false }) {
     this.cancelOnError = cancelOnError;
     subscription = stream.listen(add,
                                  onError: addError,
                                  onDone: close,
                                  cancelOnError: cancelOnError);
   }
 
-  void addError(error) {
-    super.addError(error);
+  void addError(error, [stackTrace]) {
+    super.addError(error, stackTrace);
     if (cancelOnError) {
       onDoneSignal.complete();
     }
   }
 
   void pause([Future resumeSignal]) {
     if (trace) print("Events#$hashCode: pause");
     subscription.pause(resumeSignal);
   }
 
   void resume() {
     if (trace) print("Events#$hashCode: resume");
     subscription.resume();
   }
 
   void onDone(void action()) {
     if (trace) print("Events#$hashCode: onDone");
     super.onDone(action);
   }
 }