Minified versus development scripts and .min.js
For some time (since r10291), WordPress has shipped minified scripts and styles as .js and .css, with the non-minified, “development” versions at .dev.js and .dev.css.
These weren’t great for discoverability and it has become clear that these are non-standard. So, we’ve moved to using .min.js and .min.css for minified files. You can now find the “development” versions at .js and .css. This also works nicely with tools like ack, which are coded to ignore .min.js.
This was implemented in #21633. Now if only we can get TinyMCE to move away from _src.js. 
A note, for some external libraries, we don’t include the un-minified versions. In this case, you can find them on their respective websites and also in the sources repository. (This is linked from wordpress.org, which in turn is referenced at the bottom of our license file.) @scribu and I were talking about writing a developer plugin to use un-minified versions of these libraries, which would be cool.
More on external libraries in 3.5 here.
Eric Mann 11:16 pm on August 29, 2012 Permalink
+1 on the idea of a dev plugin to use un-minified versions. Perhaps just a way to pull them automatically from the original source if SCRIPT_DEBUG is on?
Japh 12:25 am on August 30, 2012 Permalink
Perhaps you could use the existing Developer plugin? http://wordpress.org/extend/plugins/developer/
Nikolay Bachiyski 11:23 pm on August 31, 2012 Permalink
Japh, that’s pretty cool idea: https://github.com/Automattic/developer/issues/22
Ben Tremblay 2:56 am on August 30, 2012 Permalink
I like the fact that you posted this.
Skunk Works … document things, and leave your engineering journal open on your desk.
#culture
Jan Fabry 5:30 am on August 30, 2012 Permalink
I once wrote a plugin to do this for the TinyMCE files, it was not too hard. Don’t know whether the hack will work for current versions of WordPress (because TinyMCE did not use wp_register_script(), I had to use another hook).