Remove AsyncError with Expando.

tests/lib/async/stream_event_transform_test.dart

 // Copyright (c) 2011, 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 stream_event_transform_test;
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import '../../../pkg/unittest/lib/unittest.dart';
 import 'event_helper.dart';
 
 void handleData(int data, EventSink<int> sink) {
-  sink.addError(new AsyncError("$data"));
+  sink.addError("$data");
   sink.add(data + 1);
 }
 
-void handleError(AsyncError e, EventSink<int> sink) {
-  String value = e.error;
+void handleError(error, EventSink<int> sink) {
+  String value = error;
   int data = int.parse(value);
   sink.add(data);
-  sink.addError(new AsyncError("${data + 1}"));
+  sink.addError("${data + 1}");
 }
 
 void handleDone(EventSink<int> sink) {
   sink.add(99);
   sink.close();
 }
 
 class EventTransformer extends StreamEventTransformer<int,int> {
   void handleData(int data, EventSink<int> sink) {
-    sink.addError(new AsyncError("$data"));
+    sink.addError("$data");
     sink.add(data + 1);
   }
 
-  void handleError(AsyncError e, EventSink<int> sink) {
-    String value = e.error;
+  void handleError(String value, EventSink<int> sink) {
     int data = int.parse(value);
     sink.add(data);
-    sink.addError(new AsyncError("${data + 1}"));
+    sink.addError("${data + 1}");
   }
 
   void handleDone(EventSink<int> sink) {
     sink.add(99);
     sink.close();
   }
 }
 
 main() {
   {
@@ skipping 23 matching lines @@
             handleData: handleData,
             handleError: handleError,
             handleDone: handleDone
         )));
     actual.onDone(() {
       Expect.listEquals(expected.events, actual.events);
     });
     input.replay(c);
   }
 }