this should work but it doesn't :(

gistfile1.js

#!/usr/bin/gjs

const Lang = imports.lang;
const Gtk = imports.gi.Gtk;
const Gio = imports.gi.Gio;

const Application = new Lang.Class({
    Name: 'Demo async file write',

    _init: function() {
        this.application = new Gtk.Application();

        this.application.connect('startup', Lang.bind(this, this._onStartup));
        this.application.connect('activate', Lang.bind(this, this._onActivate));
    },

    _onActivate: function() {
        this._window.show_all();
    },

    _appendAsyncCb: function(obj, res, data) {
        let output_stream = obj.append_to_finish (res, null);
        output_stream.write_async("Writing asynchronously...", null,
                                 Lang.bind (this, function(obj, res, data) {
                                     obj.write_finish(res, null);
                                 }));
    },

    _buildUI: function() {
        this._window = new Gtk.ApplicationWindow({ application: this.application,
                                                   title: "Hello World!" });
        this._window.set_default_size(200, 200);
        this.label = new Gtk.Label({ label: "Hello World" });
        this._window.add(this.label);
    },

    _onStartup: function() {
        let file = Gio.File.new_for_path("/home/sciamp/async_file_write_demo.txt");

        if(!file.query_exists(null))
            file.create(Gio.FileCreateFlags.NONE, null);

        file.append_to_async (Gio.FileCreateFlags.NONE, null, null,
                              Lang.bind(this, this._appendAsyncCb));

        this._buildUI();
    }

    
});

let app = new Application();
app.application.run(ARGV);