This is a sandbox project, which contains experimental code for developer use only.
This project is an example of how contrib modules that provide custom field types can write their migrations. It's not currently finished and the architecture may improve.
I choose the iFrame module to use as an experiment: drupal.org/project/iframe. The actual migration right now moves the iFrame into a link field but it shows the required concepts.
There are a few core patches required for this module/test to work.