If you're using Pantheon's free development hosting to build your site, you'll have access to the backup tools that are part of their hosting platform. Note that automatic backups only happen if you have a paid plan, so you'll need to do your backups manually.

Pantheon's own documentation on backups is the most up-to-date.

A tip: if you're developing in SFTP mode (which is most likely, because it enables you to upload modules directly from Drupal's UI), be sure to commit your changes before doing a backup through Pantheon. Backups through the Backup and Migrate module will capture everything on your site, but any modules or themes that you've added through Drupal's UI need to be committed before Pantheon's backup will pick them up.