Fix unresolved ports negative test

tests/isolate/unresolved_ports_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.
 
 // spawns multiple isolates and sends unresolved ports between them.
 library unresolved_ports;
 import 'dart:isolate';
 import '../../pkg/unittest/lib/unittest.dart';
 
 // This test does the following:
 //  - main spawns two isolates: 'tim' and 'beth'
 //  - 'tim' spawns an isolate: 'bob'
 //  - main starts a message chain:
 //       main -> beth -> tim -> bob -> main
 //    by giving 'beth' a send-port to 'tim'.
 
 bethIsolate() {
+  // TODO(sigmund): use expectAsync2 when it is OK to use it within an isolate
+  // (issue #6856)
   port.receive((msg, reply) => msg[1].send(
         '${msg[0]}\nBeth says: Tim are you coming? And Bob?', reply));
 }
 
 timIsolate() {
   SendPort bob = spawnFunction(bobIsolate);
   port.receive((msg, reply) => bob.send(
         '$msg\nTim says: Can you tell "main" that we are all coming?', reply));
 }
 
 bobIsolate() {
   port.receive((msg, reply) => reply.send(
         '$msg\nBob says: we are all coming!'));
 }
 
-main() {
+baseTest({bool failForNegativeTest: false}) {
   test('Message chain with unresolved ports', () {
     ReceivePort port = new ReceivePort();
     port.receive(expectAsync2((msg, _) {
       expect(msg, equals('main says: Beth, find out if Tim is coming.'
         '\nBeth says: Tim are you coming? And Bob?'
         '\nTim says: Can you tell "main" that we are all coming?'
         '\nBob says: we are all coming!'));
+      expect(failForNegativeTest, isFalse);
       port.close();
     }));
 
     SendPort tim = spawnFunction(timIsolate);
     SendPort beth = spawnFunction(bethIsolate);
 
     beth.send(
         // because tim is created asynchronously, here we are sending an
         // unresolved port:
         ['main says: Beth, find out if Tim is coming.', tim],
         port.toSendPort());
   });
 }
+
+main() => baseTest();