writeAsync method
Implementation
Future<File> writeAsync([String path,File file]){
String content = this.toMDString();
if(path==null&&file!=null){
path = file.path;
}else if(path!=null&&file==null){
file = File(path);
}else{
throw ArgumentError("引数pathまたは引数fileのいずれか1つのみ指定してください");
}
return file.writeAsString(content);
}