html2canvas/src/xhr.js

function XHR(url) {
    return new Promise(function(resolve, reject) {
        var xhr = new XMLHttpRequest();
        xhr.open('GET', url);

        xhr.onload = function() {
            if (xhr.status === 200) {
                resolve(xhr.responseText);
            } else {
                reject(new Error(xhr.statusText));
            }
        };

        xhr.onerror = function() {
            reject(new Error("Network Error"));
        };

        xhr.send();
    });
}