code: boolean contains(ProtoMember protoMember) {
      if (protoMember == null) throw new NullPointerException("protoMember == null");
      if (identifierSet.excludes(protoMember)) return false;
      return members.containsKey(protoMember.type())
          ? members.containsEntry(protoMember.type(), protoMember)
          : types.contains(protoMember.type());
    }
docs: Returns true if {@code member} is marked and should be retained.