Remove language version from spawnHybrid code (dart-lang/web_socket_channel#232)

The code is compatible with null safety without any changes.
diff --git a/pkgs/web_socket_channel/test/html_test.dart b/pkgs/web_socket_channel/test/html_test.dart
index 3a89098..f7ec4d9 100644
--- a/pkgs/web_socket_channel/test/html_test.dart
+++ b/pkgs/web_socket_channel/test/html_test.dart
@@ -16,7 +16,6 @@
   late int port;
   setUpAll(() async {
     final channel = spawnHybridCode(r'''
-      // @dart=2.7
       import 'dart:io';
 
       import 'package:stream_channel/stream_channel.dart';