Update .gitignore to new `dart_tool` pub cache (#162)

dart-lang/sdk#32030

.gitignore

diff --git a/.gitignore b/.gitignore
index 8b7331f..b4ad53c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,6 +4,7 @@
 .buildlog
 .packages
 .project
+.dart_tool/
 .pub/
 build/
 **/packages/