Update .gitignore to new `dart_tool` pub cache (#19)

dart-lang/sdk#32030
diff --git a/.gitignore b/.gitignore
index 52f0c5b..9c55169 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
 .packages
+.dart_tool/
 .pub/
 pubspec.lock