flutter dart: how to encode a map / dictionary to JSON - json

Question

I managed to decode a JSON dictionary but how can I do the opposite - encoding?
class Achievements {
Achievements({this.skills});
var skills = Map<String, SkillHist>();
// ==================================================ENCODING DICTIONARY
factory Achievements.fromJson(Map<String, dynamic> json) {
var innerMap = json['skills'];
var skillMap = Map<String, SkillHist>();
innerMap.forEach((key, value) {
skillMap.addAll({key: SkillHist.fromJson(value)});
});
return Achievements(
skills: skillMap,
);
}
// ==================================================DECODING DICTIONARY
Map<String, dynamic> toJson() => {
"skills": skills == null ? null : Map // <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
};
}
class SkillHist {
SkillHist({this.currentSkill, this.skillLevel, this.skillEarnedDate});
final int currentSkill;
final List<int> skillLevel;
final List<DateTime> skillEarnedDate;
factory SkillHist.fromJson(Map<String, dynamic> json) {
return SkillHist(
currentSkill: json["currentSkill"] == null ? null : json['currentSkill'],
skillLevel: json["skillLevel"] == null ? null : List<int>.from(json["skillLevel"].map((x) => x)),
skillEarnedDate: json["skillEarnedDate"] == null ? null : List<DateTime>.from(json["skillEarnedDate"].map((x) => x)),
);
}
Map<String, dynamic> toJson() => {
"currentSkill": currentSkill == null ? null : currentSkill,
"skillLevel": skillLevel == null ? null : List<int>.from(skillLevel.map((x) => x)),
"skillEarnedDate": skillEarnedDate == null ? null : List<DateTime>.from(skillEarnedDate.map((x) => x)),
};
}
With this issue in mind, I also started wondering how I implemented the decoding of a List, which I copied from some snippet, months ago.
Why am I using
"skillEarnedDate": List<DateTime>.from(skillEarnedDate.map((x) => x)),
and not simply
"skillEarnedDate": skillEarnedDate,
skillEarnedDate is a List already, so why do I have initiate a new one?