Remove AsyncError with Expando.

tests/standalone/io/echo_server_stream_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.
 //
 // Echo server test program to test socket streams.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
@@ skipping 39 matching lines @@
       Expect.equals(MSGSIZE, offset);
       _messages++;
       if (_messages < MESSAGES) {
         sendData();
       } else {
         shutdown();
       }
     }
 
     void errorHandler(e) {
-      Expect.fail("Socket error $e");
+      String msg = "Socket error $e";
+      var trace = getAttachedStackTrace(e);
+      if (trace != null) msg += "\nStackTrace: $trace";
+      Expect.fail(msg);
     }
 
     void connectHandler() {
       _socket.listen(onData,
                      onError: errorHandler,
                      onDone: onClosed);
       _socket.add(_buffer);
       _socket.close();
       data = new List<int>(MSGSIZE);
     }
@@ skipping 50 matching lines @@
           Expect.equals(EchoServerGame.FIRSTCHAR + i, buffer[i]);
         }
         if (offset == MSGSIZE) {
           connection.add(buffer);
           connection.close();
         }
       }
     }
 
     void errorHandler(e) {
-      Expect.fail("Socket error $e");
+      String msg = "Socket error $e";
+      var trace = getAttachedStackTrace(e);
+      if (trace != null) msg += "\nStackTrace: $trace";
+      Expect.fail(msg);
     }
 
     connection.listen(dataReceived, onError: errorHandler);
   }
 }
 
 main() {
   EchoServerGame echoServerGame = new EchoServerGame.start();
 }