Update .gitignore to new `dart_tool` pub cache (#31)

dart-lang/sdk#32030
diff --git a/.gitignore b/.gitignore
index 9bd4f34..f73b2f9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,4 @@
-packages
-pubspec.lock
-/.pub
+.dart_tool/
 .packages
+.pub/
+pubspec.lock