compile() {
    var css = super.compile()
      , out = this.basename + '.map'
      , url = this.normalizePath(this.dest
        ? join(this.dest, out)
        : join(dirname(this.filename), out))
      , map;

    if (this.inline) {
      map = this.map.toString();
      url = 'data:application/json;'
        + (this.utf8 ? 'charset=utf-8;' : '') + 'base64,'
        + Buffer.from(map).toString('base64');
    }
    if (this.inline || false !== this.comment)
      css += '/*# sourceMappingURL=' + url + ' */';
    return css;
  }
Generate and write source map.

@return {String}
@api private