Update .gitignore to new `dart_tool` pub cache (dart-lang/web_socket_channel#19)

dart-lang/sdkdart-lang/web_socket_channel#32030

pkgs/web_socket_channel/.gitignore

diff --git a/pkgs/web_socket_channel/.gitignore b/pkgs/web_socket_channel/.gitignore
index 52f0c5b..9c55169 100644
--- a/pkgs/web_socket_channel/.gitignore
+++ b/pkgs/web_socket_channel/.gitignore
@@ -1,3 +1,4 @@
 .packages
+.dart_tool/
 .pub/
 pubspec.lock