PK identification causes incorrect unmapped flag

Migrate UI runs the following code to show all source fields:

      if (isset($source_key[$machine_name])) {
        // Identify primary key
        $machine_name .= ' ' . t('(PK)');
      }
      else {
        // Add class for mapped/unmapped. Used in summary.
        $classes = !isset($source_fields[$machine_name]) ? 'migrate-error' : '';
      }
      $rows[] = array(
        array('data' => check_plain($machine_name), 'class' => $classes),
        array('data' => filter_xss_admin($description), 'class' => $classes),
      );

Compare this to the code to show all destination fields:

      $classes = array();
      if (isset($dest_key[$machine_name])) {
        // Identify primary key
        $machine_name .= ' ' . t('(PK)');
      }
      else {
        // Add class for mapped/unmapped. Used in summary.
        $classes[] = !isset($destination_fields[$machine_name]) ? 'migrate-error' : '';
      }
      $rows[] = array(
        array('data' => check_plain($machine_name), 'class' => $classes),
        array('data' => filter_xss_admin($description), 'class' => $classes),
      );

The former causes the PK field to show as unmapped when the previous field was unmapped because $classes isn't reset. The latter at least resets the $classes variable (and correctly uses it as an array!), but still does not account for an unmapped PK.

Attached is a patch that makes both pieces of code flag an unmapped PK correctly.