static caseLessAlphaComparator_(a, b) {
    if (a.toLowerCase() < b.toLowerCase()) {
      return -1;
    }
    if (a.toLowerCase() > b.toLowerCase()) {
      return 1;
    }
    return 0;
  }
A sort comparator for comparing two messages, ignoring case.
@param {string} a
@param {string} b
@return {number}
@private