main.dart

class Person {
  const Person(this.name, this.age);
  final String name;
  final int age;

  factory Person.fromJson(Map<String, Object> json) {
    final name = json['name'];
    final age = json['age'];

    if (name == null) {
      throw UnimplementedError('Key name is not implemented');
    }
    if (age == null) {
      throw UnimplementedError('Key age is not implemented');
    }

    if (name is String && age is int) {
      return Person(name, age);
    }
    throw UnsupportedError('Invalid type name or age');
  }

  Map<String, Object> toJson() => {'name': name, 'age': age};
}

void main() {
  var person = Person.fromJson({
    'name': 'Lucy',
    'age': 48,
  });
  var json = person.toJson();
  print(json);
}

Verison 2 here. Lazy coders, error exception is shortor.

class Person {
  const Person({required this.name, required this.age});
  final String name;
  final int age;

  factory Person.fromJson(Map<String, Object> json) {
    final name = json['name'];
    final age = json['age'];

    if (name is String && age is int) {
      return Person(name: name, age: age);
    }
    throw UnsupportedError('Invalid key name or value type');
  }

  Map<String, Object> toJson() => {'name': name, 'age': age};
}

void main() {
  var person = Person.fromJson({
    'name': 'Lucy',
    'age': 48,
  });
  var json = person.toJson();
  print(json);
}