Fixing broken URL Aliases on a Drupal 6 -> Drupal 8 migration

So there are definite "gotchas" to migrating content from Drupal 6 to Drupal 8, when you take away the assumption that the ids throughout the system will remain the same. We hit another one on a recent launch: the URL aliases imported from Drupal 6 did not get rewritten with new node ids, after the migration had started using a map.