src/windows/SyncProxy.js
function startDownload(src, storageFile) {
    var uri = Windows.Foundation.Uri(src);
    var downloader = new Windows.Networking.BackgroundTransfer.BackgroundDownloader();
    var download = downloader.createDownload(uri, storageFile);
    return download.startAsync();
}
function startDownload(src, storageFile) {
    var uri = Windows.Foundation.Uri(src);
    var downloader = new Windows.Networking.BackgroundTransfer.BackgroundDownloader();
    var download = downloader.createDownload(uri, storageFile);
    return download.startAsync();
}
this can throw exceptions, callers responsibility