anchor.diff:        "emails": emails,
         "phones": phones,
         "postalAddresses": postalAddresses,
  -      "avatar": contact.avatar
  +      "avatar": contact.avatar,
  +      "birthday": birthday
       };
     }
   
  
positive.code:     postalAddresses = (m["postalAddresses"] as Iterable)
          ?.map((m) => PostalAddress.fromMap(m));
      avatar = m["avatar"];
    }
  
    static Map _toMap(Contact contact) {
  
negative.code:       this.jobTitle,
        this.middleName,
        this.prefix,
        this.suffix,
        this.birthday,
      ].where((s) => s != null));
    }
  }
  