Remove AsyncError with Expando.

tests/standalone/io/testing_server.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.
 
 part of ServerTest;
 
 abstract class TestingServer {
 
   static const HOST = "127.0.0.1";
   static const INIT = 0;
   static const SHUTDOWN = -1;
 
   void onConnection(Socket connection);  // Abstract.
 
   void errorHandlerServer(e) {
-    Expect.fail("Server socket error $e");
+    String msg = "Server socket error $e";
+    var trace = getAttachedStackTrace(e);
+    if (trace != null) msg += "\nStackTrace: $trace";
+    Expect.fail(msg);
   }
 
   void dispatch(message, SendPort replyTo) {
     if (message == INIT) {
       ServerSocket.bind(HOST, 0, 10).then((server) {
         _server = server;
         _server.listen(
             onConnection,
             onError: errorHandlerServer);
         replyTo.send(_server.port, null);
@@ skipping 27 matching lines @@
 
   void shutdown() {
     _sendPort.send(TestingServer.SHUTDOWN, _receivePort.toSendPort());
     _receivePort.close();
   }
 
   int _port;
   ReceivePort _receivePort;
   SendPort _sendPort;
 }