This post describes how to do it: http://myownhomeserver.com/2011/10/wordpress-addremove-quicktags-simple-editor-buttons-in-3-3/  

WordPress minifies the core javascript files.  For instance it uses editor.js instead of the original source code file editor.dev.js.  You can’t just rename the .dev.js files as the minifying also changes things such as variable names.  However you can tell wordpress to use the dev.js files instead by adding this to wp-config.php (before any requires): define(‘SCRIPT_DEBUG’, true);