Use wp-less to process editor stylesheets.

gistfile1.php

<?php
add_filter('mce_css', 'theme_mce_less');
function theme_mce_less($css)
{
  $handle = 'editor';
  $less_plugin = WPLessPlugin::getInstance();
  wp_enqueue_style($handle, get_stylesheet_directory_uri()."/assets/stylesheets/editor.less");
  
  $less_plugin->processStylesheets();
  global $wp_styles;
  $src = $wp_styles->registered[$handle]->src;
  wp_dequeue_style($handle);
  return $css ? $css.','.$src : $src;
}