Add tinymce editor for post excerpt

editor-post-excerpt.php

<?php
add_action( 'add_meta_boxes', array ( 'add_tinymce_to_excerpt', 'switch_boxes' ) );
class add_tinymce_to_excerpt{
    public static function switch_boxes(){
        if ( ! post_type_supports( $GLOBALS['post']->post_type, 'excerpt' ) ){
            return;
        }
        remove_meta_box( 'postexcerpt', '', 'normal' );
        add_meta_box( 'postexcerpt2', 'Excerpt' , array ( __CLASS__, 'show' ), null, 'normal', 'core' );
    }

    public static function show( $post ){
    ?>
        <label class="screen-reader-text" for="excerpt">Excerpt</label>
        <?php
        wp_editor( self::unescape( $post->post_excerpt ), 'excerpt', array ( 'textarea_rows' => 15, 'media_buttons' => false, 'teeny' => false, 'tinymce' => true ) );
    }
    public static function unescape( $str ){
        return str_replace( array ( '&lt;', '&gt;', '&quot;', '&amp;', '&nbsp;', '&amp;nbsp;' ), array ( '<', '>', '"', '&', ' ', ' ' ), $str);
    }
}