cd ~/Sites/Dev Desktop/
rm -rf ./drupal8

git clone --branch 8.x http://git.drupal.org/project/drupal.git ./drupal8

username: drupaluser
password: <blank>

port: 33067
localhost: 127.0.0.1

$ mkdir path/to/your/new/docroot
$ cd path/to/your/new/docroot
$ drush dl drupal-8.x