Regression: MigrateDestinationEntityAPI import fails when system-of-record is destination

At the top of MigrateDestinationEntityAPI::import(), there's a check on whether the entity was previously migrated. If not, the entity ID is unset.

  public function import(stdClass $entity, stdClass $row) {
    $migration = Migration::currentMigration();

    // Updating previously-migrated content?
    if (isset($row->migrate_map_destid1)) {
      if (isset($entity->{$this->id})) {
        if ($entity->{$this->id} != $row->migrate_map_destid1) {
          throw new MigrateException(t("Incoming id !id and map destination id !destid1 don't match",
            array('!id' => $entity->{$this->id}, '!destid1' => $row->migrate_map_destid1)));
        }
      }
      else {
        $entity->{$this->id} = $row->migrate_map_destid1;
      }
    }
    else {
      unset($entity->{$this->id});
    }

This is a problem if our system of record is Migration::DESTINATION, which throws a "no destination id provided" error.

Looking through the git history, it looks like this was introduced in #1530848: Entity API plugin: various cleanups and improvements.