Swallow errors in subscriptions in http_multi_server..

pkg/http_multi_server/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.
 
 library http_multi_server.test;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:http/http.dart' as http;
@@ skipping 90 matching lines @@
 
     test("listens on all localhost interfaces", () {
       server.listen((request) {
         request.response.write("got request");
         request.response.close();
       });
 
       expect(http.read("http://127.0.0.1:${server.port}/"),
           completion(equals("got request")));
 
-      return supportsIpV6.then((supportsIpV6) {
+      return _supportsIpV6.then((supportsIpV6) {
         if (!supportsIpV6) return;
         expect(http.read("http://[::1]:${server.port}/"),
             completion(equals("got request")));
       });
     });
   });
 }
 
+/// A cache for [supportsIpV6].
+bool _supportsIpV6Cache;
+
+// TODO(nweiz): This is known to be inaccurate on Windows machines with IPv6
+// disabled (issue 19815). Tests will fail on such machines.
+/// Returns whether this computer supports binding to IPv6 addresses.
+Future<bool> get _supportsIpV6 {
+  if (_supportsIpV6Cache != null) return new Future.value(_supportsIpV6Cache);
+
+  return ServerSocket.bind(InternetAddress.LOOPBACK_IP_V6, 0).then((socket) {
+    _supportsIpV6Cache = true;
+    socket.close();
+    return true;
+  }).catchError((error) {
+    if (error is! SocketException) throw error;
+    _supportsIpV6Cache = false;
+    return false;
+  });
+}
+
 /// 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}/";