Change '127.0.0.1' to 'localhost' to support both IPv4 and IPv6 envs.

test/http_multi_server_test.dart

 // Copyright (c) 2014, 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 'package:http/http.dart' as http;
 import 'package:http_multi_server/http_multi_server.dart';
 import 'package:http_multi_server/src/utils.dart';
 import 'package:test/test.dart';
 
 void main() {
   group("with multiple HttpServers", () {
     var multiServer;
     var subServer1;
     var subServer2;
     var subServer3;
     setUp(() {
       return Future.wait([
-        HttpServer.bind("127.0.0.1", 0).then((server) => subServer1 = server),
-        HttpServer.bind("127.0.0.1", 0).then((server) => subServer2 = server),
-        HttpServer.bind("127.0.0.1", 0).then((server) => subServer3 = server)
+        HttpServer.bind("localhost", 0).then((server) => subServer1 = server),
+        HttpServer.bind("localhost", 0).then((server) => subServer2 = server),
+        HttpServer.bind("localhost", 0).then((server) => subServer3 = server)
       ]).then((servers) => multiServer = new HttpMultiServer(servers));
     });
 
     tearDown(() => multiServer.close());
 
     test("listen listens to all servers", () {
       multiServer.listen((request) {
         request.response.write("got request");
         request.response.close();
       });
@@ skipping 110 matching lines @@
       server.listen((request) {
         request.response.write("got request");
         request.response.close();
       });
 
       if (await supportsIPv4) {
         expect(http.read("http://127.0.0.1:${server.port}/"),
             completion(equals("got request")));
       }
 
-      if (await supportsIPv4) {
+      if (await supportsIPv6) {
         expect(http.read("http://[::1]:${server.port}/"),
             completion(equals("got request")));
       }
     });
   });
 }
 
 /// Makes a GET request to the root of [server] and returns the response.
 Future<http.Response> _get(HttpServer server) => http.get(_urlFor(server));
 
 /// Makes a GET request to the root of [server] and returns the response body.
 Future<String> _read(HttpServer server) => http.read(_urlFor(server));
 
 /// Returns the URL for the root of [server].
 String _urlFor(HttpServer server) =>
     "http://${server.address.host}:${server.port}/";