write the body of the HTTP
request. When one of the IOSink methods is used for the first
time, the request header is sent. Calling any methods that
change the header after it is sent throws an exception.</p>
<p>When writing string data through the <a href="dart-io/IOSink-class.html">IOSink</a> the
encoding used is determined from the "charset" parameter of
the "Content-Type" header.</p>
<pre class="language-dart"><code>HttpClientRequest request = ...
request.headers.contentType
    = new ContentType("applic