Fix analyser warnings

tests/standalone/io/http_bind_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.
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:convert';
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
@@ skipping 57 matching lines @@
     }
     var futures = [];
     for (var host in addresses) {
       futures.add(testBindShared(host, false));
       futures.add(testBindShared(host, true));
     }
     Future.wait(futures).then((_) => asyncEnd());
   });
 }
 
-bool supportsIPV6() async {
+Future<bool> supportsIPV6() async {
   try {
     var socket = await ServerSocket.bind('::1', 0);
     await socket.close();
     return true;
   } catch (e) {
     print(e);
     return false;
   }
 }