Remove AsyncError with Expando.

tests/standalone/io/socket_many_connections_test.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.
 //
 // Test creating a large number of socket connections.
 library ServerTest;
 
 import "package:expect/expect.dart";
 import "dart:io";
 import "dart:isolate";
@@ skipping 62 matching lines @@
 class TestServer extends TestingServer {
 
   void onConnection(Socket connection) {
     Socket _client;
 
     void closeHandler() {
       connection.close();
     }
 
     void errorHandler(e) {
-      print("Socket error $e");
+      String msg = "Socket error $e";
+      var trace = getAttachedStackTrace(e);
+      if (trace != null) msg += "\nStackTrace: $trace";
+      print(msg);
       connection.close();
     }
 
     _connections++;
     connection.listen(
         (data) {},
         onDone: closeHandler,
         onError: errorHandler);
   }
 
   int _connections = 0;
 }
 
 main() {
   SocketManyConnectionsTest test = new SocketManyConnectionsTest.start();
 }