Ignore unused fields in private class – copied from SDK

pkgs/web_socket_channel/lib/src/copy/web_socket_impl.dart

diff --git a/pkgs/web_socket_channel/lib/src/copy/web_socket_impl.dart b/pkgs/web_socket_channel/lib/src/copy/web_socket_impl.dart
index 21464a2..b980bec 100644
--- a/pkgs/web_socket_channel/lib/src/copy/web_socket_impl.dart
+++ b/pkgs/web_socket_channel/lib/src/copy/web_socket_impl.dart
@@ -12,6 +12,8 @@
 // This is up-to-date as of sdk revision
 // 365f7b5a8b6ef900a5ee23913b7203569b81b175.
 
+// ignore_for_file: unused_field
+
 import 'dart:async';
 import 'dart:convert';
 import 'dart:math';