Update .gitignore to new `dart_tool` pub cache

dart-lang/sdk#32030
diff --git a/.gitignore b/.gitignore
index 7dbf035..813a31e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,7 @@
 # Don’t commit the following directories created by pub.
 .buildlog
 .pub/
+.dart_tool/
 build/
 packages
 .packages
@@ -12,4 +13,4 @@
 *.js.map
 
 # Include when developing application packages.
-pubspec.lock
\ No newline at end of file
+pubspec.lock