Remove AsyncError with Expando.

tests/standalone/io/http_server_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 "dart:io";
 import "dart:isolate";
 
 void testListenOn() {
   ServerSocket socket;
@@ skipping 11 matching lines @@
       })
       .then((response) {
         response.listen(
           (_) {},
           onDone: () {
             client.close();
             clientPort.close();
             onDone();
           });
       })
-      .catchError((error) {
-        Expect.fail("Unexpected error in Http Client: $error");
+      .catchError((e) {
+        String msg = "Unexpected error in Http Client: $e";
+        var trace = getAttachedStackTrace(e);
+        if (trace != null) msg += "\nStackTrace: $trace";
+        Expect.fail(msg);
       });
   }
 
   // Test two connection after each other.
   ServerSocket.bind().then((s) {
     socket = s;
     server = new HttpServer.listenOn(socket);
     ReceivePort serverPort = new ReceivePort();
     server.listen((HttpRequest request) {
       request.listen(
@@ skipping 10 matching lines @@
         Expect.throws(() => server.port);
         socket.close();
       });
     });
   });
 }
 
 void main() {
   testListenOn();
 }