headAjax function
Sends an Ajax request to the given url using the HEAD method.
Unlike ajax, the returned http.Response.contentLength reflects
the wire's Content-Length header — http.Response.fromStream
would otherwise report 0 for HEAD (the body is empty by
definition).
Implementation
Future<http.Response> headAjax(Uri url, {
Map<String, String>? headers,
Duration? timeout})
=> _ajax(url, method: "HEAD", headers: headers, timeout: timeout,
getResponse: _headResponse);