There is no explicit documentation for the Drupal 8 version of the flag module, and now that the Drupal 8 version has hit alpha, it's a good time to share some information on it.

In Drupal 8, migration destinations are easy to set up for entities. Since flags are entities in Drupal 8, all you need to do is set up a standard entity migration configuration with the fields for creating a flag. 

The fields are:

flag_id - The flag's bundle machine name in Drupal