Relax vmservice isolate tests.

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:io';
 import 'dart:json' as JSON;
 import 'dart:utf' as UTF;
@@ skipping 142 matching lines @@
     });
     Expect.isTrue(exists, 'No isolate with name: $name');
   }
 
   void checkIsolateNamePrefix(int id, String name) {
     var exists = false;
     isolateList['members'].forEach((isolate) {
       if (isolate['id'] == id) {
         exists = true;
         Expect.isTrue(isolate['name'].startsWith(name),
-                      'Isolate $id does not have name prefix: $name');
+                      'Isolate $id does not have name prefix: $name'
+                      ' (was ${isolate['name']})');
       }
     });
     Expect.isTrue(exists, 'No isolate with id: $id');
   }
 }