How to get last-modified from http header with dartio HttpClient - flutter

Question

Hi there I want get Last_modified from http header with dart.io HttpClient()
code sample is:
var client = new HttpClient();
HttpClientRequest req = await client.getUrl(Uri.parse("sayagh.asnafhormozgan.ir/wp-content/tables/essentials.csv"));
var a = req.headers.value("lastModifiedHeader");
but a returns null;
how I can get Last modified?
but when I get it with curl:
curl -v "sayagh.asnafhormozgan.ir/wp-content/tables/drawer.csv"
* Trying 51.89.173.235:80...
* TCP_NODELAY set
* Connected to sayagh.asnafhormozgan.ir (51.89.173.235) port 80 (#0)
> GET /wp-content/tables/drawer.csv HTTP/1.1
> Host: sayagh.asnafhormozgan.ir
> User-Agent: curl/7.65.3
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Server: nginx
< Date: Thu, 19 Sep 2019 10:05:37 GMT
< Content-Type: text/csv
< Content-Length: 599
< Connection: keep-alive
< Last-Modified: Thu, 19 Sep 2019 09:38:30 GMT
< Accept-Ranges: bytes
< Cache-Control: max-age=0
< Expires: Thu, 19 Sep 2019 10:05:37 GMT
<

Answer 1

You'll need to wait for the second future to get the response, i.e.:
A getUrl request is a two-step process, triggered by two Futures. When
the first future completes with a HttpClientRequest, the underlying
network connection has been established, but no data has been sent. In
the callback function for the first future, the HTTP headers and body
can be set on the request. Either the first write to the request
object or a call to close sends the request to the server.
See https://api.dartlang.org/stable/2.5.0/dart-io/HttpClient-class.html
Also it should be 'last-modified' and not 'lastModifiedHeader' (or even better use the static const variable HttpHeaders.lastModifiedHeader), e.g.:
HttpClient client = HttpClient();
HttpClientRequest req = await client.getUrl(Uri.parse(
'http://sayagh.asnafhormozgan.ir/wp-content/tables/essentials.csv'));
HttpClientResponse response = await req.close();
print(response.headers.value(HttpHeaders.lastModifiedHeader));