Fix warnings in library-tests.

tests/lib/async/stream_event_transformed_test.dart

 // Copyright (c) 2013, 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.
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'dart:async';
 import 'event_helper.dart';
 
 
 class DecrementingTransformerSink implements EventSink {
   final outSink;
   DecrementingTransformerSink(this.outSink);
 
   void add(int i) => outSink.add(i - 1);
-  void addError(int e, st) => outSink.addError(e - 1, st);
+  void addError(int e, [st]) => outSink.addError(e - 1, st);
   void close() => outSink.close();
 }
 
 class FutureWaitingTransformerSink implements EventSink {
   final outSink;
   final closeFuture;
   FutureWaitingTransformerSink(this.outSink, this.closeFuture);
 
   void add(Future future) { future.then(outSink.add); }
-  void addError(Future e, st) { e.then((val) { outSink.addError(val, st); }); }
+  void addError(Future e, [st]) { e.then((val) { outSink.addError(val, st); }); }
   void close() { closeFuture.whenComplete(outSink.close); }
 }
 
 class ZoneTransformerSink implements EventSink {
   final outSink;
   ZoneTransformerSink(this.outSink);
 
   void add(_) { outSink.add(Zone.current); }
-  void addError(_, st) { outSink.add(Zone.current); }
+  void addError(_, [st]) { outSink.add(Zone.current); }
   void close() {
     outSink.add(Zone.current);
     outSink.close();
   }
 }
 
 class TypeChangingSink implements EventSink<int> {
   final EventSink<String> outSink;
   TypeChangingSink(this.outSink);
 
   void add(int data) { outSink.add(data.toString()); }
-  void addError(error, st) { outSink.addError(error, st); }
+  void addError(error, [st]) { outSink.addError(error, st); }
   void close() { outSink.close(); }
 }
 
 class SinkTransformer<S, T> implements StreamTransformer<S, T> {
   final Function sinkMapper;
   SinkTransformer(this.sinkMapper);
 
   Stream<T> bind(Stream<S> stream) {
     return new Stream<T>.eventTransformed(stream, sinkMapper);
   }
@@ skipping 246 matching lines @@
     new Stream.fromIterable([1, 2, 3])
       .transform(new SinkTransformer<int, String>(
           (sink) => new TypeChangingSink(sink)))
       .toList()
       .then((list) {
         Expect.listEquals(["1", "2", "3"], list);
         asyncEnd();
       });
   }
 }