write_to_file.dart

import 'dart:io';
import 'dart:async';

Future createFile(String fileName, String text){
    return new File(fileName).create()
            .then((File file){
                file.writeAsStringSync(text);
                return 'the file has been created with "$text" as its content';
            });
}

void main(){
    final String text = "Dart is fun!";
    createFile('test.txt', text)
            .then((value) => print(value))
            .catchError((e) => print(e.toString()));
}