code: function consumeEscape (css, start) {
      let next = start;
      let code = css.charCodeAt(next + 1);
      if (unescapable[code]) {
          // just consume the escape char
      } else if (hex[code]) {
          let hexDigits = 0;
          // consume up to 6 hex chars
          do {
              next++;
              hexDigits++;
              code = css.charCodeAt(next + 1);
          } while (hex[code] && hexDigits < 6);
          // if fewer than 6 hex chars, a trailing space ends the escape
          if (hexDigits < 6 && code === t.space) {
              next++;
          }
      } else {
          // the next char is part of the current word
          next++;
      }
      return next;
  }
docs: Returns the last index of the escape sequence
  @param {string} css The string in which the sequence begins
  @param {number} start The index into the string where escape character (`\`) occurs.