PHP Notices raised by cssconcat

[westonruter]

When running unit tests for Customize Widgets Plus inside of Quickstart, I'm noticing PHP Notices being raised:

Only variable references should be returned by reference

/srv/www/wp-content/mu-plugins/http-concat/cssconcat.php:174
/srv/www/wp/wp-includes/class.wp-dependencies.php:219
/srv/www/wp/wp-includes/script-loader.php:653
/srv/www/wp/wp-includes/plugin.php:579
/srv/www/wp/wp-includes/class.wp-styles.php:39
/srv/www/wp-content/mu-plugins/http-concat/cssconcat.php:31
/srv/www/wp-content/mu-plugins/http-concat/cssconcat.php:191
/srv/www/wp/wp-includes/plugin.php:503
/srv/www/wp-tests/tests/phpunit/tests/customize/widgets.php:53
/srv/www/wp-tests/tests/phpunit/tests/customize/widgets.php:175

The code in question is:

class WPcom_CSS_Concat extends WP_Styles {
    // ...
    function &__get( $key ) {
        return $this->old_styles->$key;
    }
    // ...
}

I tried changing this method to:

    function &__get( $key ) {
        $value = $this->old_styles->$key;
        return $value;
    }

But then of course a different error is raised when wp_styles()->registered[ $key ] is attempted to be set:

Indirect modification of overloaded property WPcom_CSS_Concat::$registered has no effect

/srv/www/wp/wp-includes/class.wp-dependencies.php:221
/srv/www/wp/wp-includes/functions.wp-styles.php:116
/srv/www/wp-content/plugins/jetpack/_inc/genericons.php:11
/srv/www/wp/wp-includes/plugin.php:503
/srv/www/wp-tests/tests/phpunit/tests/customize/widgets.php:53
/srv/www/wp-tests/tests/phpunit/tests/customize/widgets.php:175

I think perhaps the approach for extending WP_Styles needs to be changed. Instead of storing the original instance of WP_Styles inside of the WPcom_CSS_Concat instance, I think it should be fully re-using all of its properties directly.