main function
void
main()
Implementation
void main() async {
List<PolygonField> polygonFields = [];
final jsonData = await File('./polygon.json').readAsString().then((fileContents) => json.decode(fileContents));
for (var elem in jsonData) {
polygonFields.add(PolygonField.fromJson(Map<String, dynamic>.from(elem)));
}
print(polygonFields);
}