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

dart-lang/sdk#32030
diff --git a/.gitignore b/.gitignore
index 98fed01..49ce72d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,3 @@
+.dart_tool/
 .packages
-packages
 pubspec.lock
-
-# IntelliJ project.
-.idea