Unless I'm mistaken, this is part of the Extension Library, so you won't find the JS file anywhere in your application.
Are you using local preview? If you are, you need to make sure that the ExtLib is not just installed into your designer, but also your local preview. Installation instructions for ExtLib should include that step.
If this is being run on the server, have there been any recent changes made? For example upgrading the server version without upgrading the ExtLib version?