Fix checked mode failure

tests/standalone/vmservice/test_helper.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.
 
 library vmservice_test_helper;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 import 'package:expect/expect.dart';
@@ skipping 201 matching lines @@
   }
 }
 
 class FieldRequestHelper extends VmServiceRequestHelper {
   FieldRequestHelper(port, isolate_id, field_id) :
       super('http://127.0.0.1:$port/isolates/$isolate_id/objects/$field_id');
   Map field;
   onRequestCompleted(Map reply) {
     Expect.equals('Field', reply['type']);
     field = reply;
-    return this;
+    return new Future.value(this);
   }
 }
 
 class ClassFieldRequestHelper extends VmServiceRequestHelper {
   final List<String> fieldNames;
   int port_;
   int isolate_id_;
   ClassFieldRequestHelper(port, isolate_id, class_id, this.fieldNames) :
       super('http://127.0.0.1:$port/isolates/$isolate_id/classes/$class_id') {
     port_ = port;
@@ skipping 14 matching lines @@
       });
     });
     return Future.wait(requests).then((a) {
       a.forEach((FieldRequestHelper field) {
         fields[field.field['user_name']] = field.field;
       });
       return this;
     });
   }
 }