Remove the workaround for issue 19815 in http_multi_server.

pkg/http_multi_server/test/http_multi_server_test.dart

Index: pkg/http_multi_server/test/http_multi_server_test.dart
diff --git a/pkg/http_multi_server/test/http_multi_server_test.dart b/pkg/http_multi_server/test/http_multi_server_test.dart
index 0140ee7c8968c37947dd33ade9dd0e8b05016b66..a3d9062067bced9da28b8452eccec18c0212a906 100644
--- a/pkg/http_multi_server/test/http_multi_server_test.dart
+++ b/pkg/http_multi_server/test/http_multi_server_test.dart
@@ -108,7 +108,7 @@ void main() {
       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")));
@@ -117,26 +117,6 @@ void main() {
   });
 }
 
-/// 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));