Updating migration without destroying existing menu links

In the prepare method, the $node looks like:

stdClass Object
(
    [body] => Array
        (
            [und] => Array
                (
                    [0] => Array
                        (
                            [value_format] => full_html
                            [format] => full_html
                            [value] => ...
                        )
                )
        )
    [uid] => 149
    [title] => Budget Reports
    [nid] => 5263
    [changed] => 2015-02-12 19:43:53
    [status] => 1
    [created] => 1328306714
    [vid] => 5437
    [migrate] => Array
        (
            [machineName] => OperationsUpdate
        )
    [path] => Array
        (
            [pathauto] => 0
            [alias] =>
        )
)

And, for what it's worth, $this->systemOfRecord = 2.

I saw the other ticket earlier with the prepare() trick, but as you can see, $node->menu isn't there, so the expression is evaluating as false.

Looking at \MigrateDestinationNode::import I'm not seeing where the menu links get assigned to $node.

migrate/plugins/destinations/node.inc:158:

  if ($migration->getSystemOfRecord() == Migration::DESTINATION) {
      if (!isset($node->nid)) {
        throw new MigrateException(t('System-of-record is DESTINATION, but no destination nid provided'));
      }
      $old_node = node_load($node->nid);
      if (empty($old_node)) {
        throw new MigrateException(t('System-of-record is DESTINATION, but node !nid does not exist',
                                   array('!nid' => $node->nid)));
      }
      if (!isset($node->created)) {
        $node->created = $old_node->created;
      }
      if (!isset($node->vid)) {
        $node->vid = $old_node->vid;
      }
      if (!isset($node->status)) {
        $node->status = $old_node->status;
      }
      if (!isset($node->uid)) {
        $node->uid = $old_node->uid;
      }
    }
    elseif (!isset($node->type)) {
      // Default the type to our designated destination bundle (by doing this
      // conditionally, we permit some flexibility in terms of implementing
      // migrations which can affect more than one type).
      $node->type = $this->bundle;
    }

So I see $node->created, $node->vid, $node->status, and $node->uid being assigned, but not $node->menu_node_links. Is that right?