Update .gitignore to new dart_tool pub cache

dart-lang/sdk#32030

Author: bcko

third_party/packages/cupertino_icons/.gitignore

@@ -2,7 +2,7 @@
 
 # Files and directories created by pub
 .packages
-.pub/
+.dart_tool
 build/
 # If you're building an application, you may want to check-in your pubspec.lock
 pubspec.lock