Yeah, this minifier is present only in the development version of WordPress and it is not shipped with code that you can find in releases. They usually ship un-minified CSS and JS, with suffix
.min for minified files. Also, based on a config option you can select which one to load between the two.
By development version of WordPress I mean the code resides in the SVN trunk (can be found here). They also maintain a GIT mirror of the SVN repo, just for people who prefer using it.
One can play with their latest trunk by cloning the git repo locally:
git clone git://develop.git.wordpress.org/ wordpress-trunk
# Gruntfile.js is the place where can one look at the
# logic the core uses to build assets with