Remove AsyncError with Expando.

tests/standalone/io/web_socket_protocol_processor_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.
 
 import "package:expect/expect.dart";
 import "dart:utf";
 import "dart:math";
 import "dart:async";
 import "dart:collection";
 import "dart:typeddata";
@@ skipping 33 matching lines @@
     Expect.listEquals(expectedMessage, buffer);
     messageCount++;
     data = buffer;
   }
 
   void onClosed() {
     closeCount++;
   }
 
   void onError(e) {
-    Expect.fail("Unexpected error $e");
+    String msg = "Unexpected error $e";
+    var trace = getAttachedStackTrace(e);
+    if (trace != null) msg += "\nStackTrace: $trace";
+    Expect.fail(msg);
   }
 
 }
 
 
 // Web socket constants.
 const int FRAME_OPCODE_TEXT = 1;
 const int FRAME_OPCODE_BINARY = 2;
 
 
@@ skipping 174 matching lines @@
       true, FRAME_OPCODE_BINARY, null, message, 0, message.length);
   controller.add(frame);
 }
 
 
 void main() {
   testFullMessages();
   testFragmentedMessages();
   testUnmaskedMessage();
 }