Migrate existing comment UIDs in a legacy migration setup.

I've been working on a set of custom continuous migration scripts (D6 to D7) using/extending the base migrate module. I discovered this module well after having already performed several migrations of users and content. The comment migration classes that are included in this migrate D2D module saved me a lot of time. However, I found that I couldn't migrate the original D6 source comment UIDs--essentially making the author for every comment 'admin' or another default user of my choosing.

I ended up replacing the following code in the base comment.inc file to add in a feature that allows direct 1:1 mapping of the source to destination comment UID. This option assumes that you already have users imported in (with 1:1 UIDs assigned) from the same D6 source database.

Before:

    if (isset($user_migration)) {
      $this->addFieldMapping('uid', 'uid')
           ->sourceMigration($user_migration)
           ->defaultValue($default_uid);
    }
    else {
      $this->addFieldMapping('uid')
           ->defaultValue($default_uid);
    }

After:

    if (isset($arguments['legacy_uid'])) {
      $legacy_uid = $arguments['legacy_uid'];
    }
    else {
      $legacy_uid = FALSE;
    }

    if ($legacy_uid === TRUE) {
      $this->addFieldMapping('uid', 'uid');
    }
    elseif (isset($user_migration)) {
      $this->addFieldMapping('uid', 'uid')
           ->sourceMigration($user_migration)
           ->defaultValue($default_uid);
    }
    else {
      $this->addFieldMapping('uid')
           ->defaultValue($default_uid);
    }

The new argument legacy_uid is set in the custom extended module *.module file.

My situation is likely a one-off case, but, I imagine there are others out there that could benefit from this option. I'll attach a patch in a subsequent comment for others to review.