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.