Remove AsyncError with Expando.

tests/standalone/io/raw_secure_socket_pause_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.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
 import "package:expect/expect.dart";
@@ skipping 82 matching lines @@
                 break;
               case RawSocketEvent.READ_CLOSED:
                 Expect.isTrue(body.length > 100);
                 Expect.equals(72, body[0]);
                 Expect.equals(9, body[body.length - 1]);
                 server.close();
                 break;
               default: throw "Unexpected event $event";
             }
           },
-          onError: (AsyncError a) {
-            Expect.fail("onError handler of RawSecureSocket stream hit: $a");
+          onError: (e) {
+            String msg = "onError handler of RawSecureSocket stream hit: $e";
+            var trace = getAttachedStackTrace(e);
+            if (trace != null) msg += "\nStackTrace: $trace";
+            Expect.fail(msg);
           });
     });
   });
 }