Update .gitignore to new `dart_tool` pub cache 

dart-lang/sdk#32030

.gitignore

diff --git a/.gitignore b/.gitignore
index 38548b3..ab3cb76 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
 # Don’t commit the following directories created by pub.
 .buildlog
+.dart_tool/
 .pub/
 build/
 packages