Adding custom styles to format dropdown in TinyMCE Editor

I’m trying to add some custom styles to the format dropdown that’s located in the “Kitchen Sink” portion of the TinyMCE Editor’s toolbar. So far I’ve tried to modify the language and editor template files located:

/wp-includes/js/tinymce/langs/wp-langs.php
/wp-includes/js/tinymce/themes/advanced/editor_template.js

I’ve added lines for the styles and I’ve even tried modifying the h1-h6 and pre tags in those files but no changes have been visible in the editor itself. What file/line am I supposed to edit to add styles to the dropdown?

In case you’re wondering, I basically searched those two files for every instance of ‘h1’ and modified that line with custom text and nothing took.