Fix analyser warnings

tests/standalone/io/http_bind_test.dart

Index: tests/standalone/io/http_bind_test.dart
diff --git a/tests/standalone/io/http_bind_test.dart b/tests/standalone/io/http_bind_test.dart
index 774471c91363775b8c2a110eeed815dd5f73bbea..4d9c68bd01136c8c221fe6dc5fc5feba0d4b2eb7 100644
--- a/tests/standalone/io/http_bind_test.dart
+++ b/tests/standalone/io/http_bind_test.dart
@@ -75,7 +75,7 @@ void main() {
   });
 }
 
-bool supportsIPV6() async {
+Future<bool> supportsIPV6() async {
   try {
     var socket = await ServerSocket.bind('::1', 0);
     await socket.close();