anchor.diff:      for (PostalAddress address in contact.postalAddresses ?? []) {
         postalAddresses.add(PostalAddress._toMap(address));
       }
  +
  +    final birthday = contact.birthday == null
  +        ? null
  +        : "${contact.birthday.year.toString()}-${contact.birthday.month.toString().padLeft(2, '0')}-${contact.birthday.day.toString().padLeft(2, '0')}";
  +
       return {
         "identifier": contact.identifier,
         "displayName": contact.displayName,
  
positive.code:               .toSet()
                .union(other.postalAddresses?.toSet() ?? Set())
                .toList(),
        avatar: this.avatar ?? other.avatar);
  
    /// Returns true if all items in this contact are identical.
    @override
  
negative.code: }
  
  class Contact {
    Contact({
      this.givenName,
      this.middleName,
      this.prefix,
      this.suffix,
      this.familyName,
      this.company,
      this.jobTitle,
      this.emails,
      this.phones,
      this.postalAddresses,
      this.avatar,
      this.birthday,
    });
  
    String identifier, displayName, givenName, middleName, prefix, suffix, familyName, company, jobTitle;
    Iterable<Item> emails = [];
    Iterable<Item> phones = [];
    Iterable<PostalAddress> postalAddresses = [];
    Uint8List avatar;
    DateTime birthday;
  
    String initials() {
      return ((this.givenName?.isNotEmpty == true ? this.givenName[0] : "") +
  