Update .gitignore to new `dart_tool` pub cache (#6)

dart-lang/sdk#32030
diff --git a/.gitignore b/.gitignore
index 25a1df3..efbbce1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,7 @@
 .buildlog
 .DS_Store
 .idea
+.dart_tool/
 .pub/
 .settings/
 build/