function handleResponse(xhr, callback, errback) {
                if (xhr.status >= 200 && xhr.status < 300) {
                    callback(xhr.responseText, xhr.getResponseHeader('Last-Modified'));
                }
                else if (typeof errback === 'function') {
                    errback(xhr.status, url);
                }
            }
Some of the functions assume a `this` context of the API object,
which causes it to fail when wrapped for ES6 imports.

An assumed `this` should be removed in the future.