我已经解决了以下问题

class MyContact {
  String userName = "";
  List<Phone> phones = List<Phone>();

 MyContact({this.userName,this.phones});

 Map<String,dynamic> toJson() {
 return {
  "name": userName,"phones": phones.map((phone) => phone.toJson()).toList(),};
}
}

class Phone {
String phone = "";

Phone({this.phone});

Map<String,dynamic> toJson() {
return {
  "phone": phone,};
}
}

方法更新

_updateContact() async {
var contacts = (await ContactsService.getContacts()).toList();
List<MyContact> myContacts = List<MyContact>();
contacts.forEach((contact) {
  List<Phone> phones = List<Phone>();
  contact.phones.forEach((phone) {
    phones.add(Phone(phone: phone.value));
  });
  myContacts.add(MyContact(userName: contact.displayName,phones: phones));
});

Map<String,dynamic> map = {
  'contact': myContacts.map((myContact) => myContact.toJson()).toList()
};
await Firestore.instance
    .collection("contacts")
    .document()
    .setData(map);
}

Flutter具有该大师的正式文件。建议使用“ json_serializable”，以使Dart代码由命令行自动生成

https://flutter.dev/docs/development/data-and-backend/json