function doGet(url, callback) {
    var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function(e) {
        if (xhr.readyState === 4) {
            var resp = null;

            if (xhr.status === 200) {
                resp = xhr.response;
                try {
                    resp = JSON.parse(/** @type {string} */ (resp));
                } catch (e) {
                    resp = null;
                }
            }
            callback(xhr.status, resp);
        }
    };
    xhr.open('GET', url, true);
    xhr.send(null);
}