What exact CSS minifier does WP core use?

Before:

add_action('admin_head', function(){

?>

    <style type="text/css">

    #wpadminbar #wp-admin-bar-wp-logo > .ab-item .ab-icon::before {
        content: "LOL";
        top: 2px;
    }   

    </style>

<?php 

});

After:

You may also use admin_enqueue_scripts action hook:

function my_enqueue($hook) {

    wp_enqueue_style( 'my_custom_script', plugin_dir_url( __FILE__ ) . 'my-style.css' );
}

add_action( 'admin_enqueue_scripts', 'my_enqueue' );

See:

• https://codex.wordpress.org/Plugin_API/Action_Reference/admin_enqueue_scripts

…didn’t even have to hack a core file.

That should get you on your way.

Regarding your comment about “My Sites” for multi-site:

What did I change?

#wpadminbar .quicklinks li .blavatar::before {
    content: "??";
    display: inline-block;
    height: 16px;
    margin: 6px 8px 0 -2px;
    width: 16px;
}

All you need to do is inspect the DOM using Chrome Dev tools or Firebug/Web Inspector in Firefox depending on your preferred method, find the selectors responsible and modify to suit your needs.

You would add the above snippet to the example showed earlier.