function tagComparator(fieldA, fieldB) {
    return fieldA.getTag() - fieldB.getTag();
  }
@param {!goog.proto2.FieldDescriptor} fieldA First field.
@param {!goog.proto2.FieldDescriptor} fieldB Second field.
@return {number} Negative if fieldA's tag number is smaller, positive
    if greater, zero if the same.