title: Flutter: How to use StreamBuilder with ListView.separated
body: <p>I want a ListView with items from a Stream.
  Of course, content of the List should reflect changes in the Stream.
  And due to my designer quirks, I want items in the List be separated by a divider.</p>
  
  <p>Just wondering, what is the proper way to create a ListView with separators, and reacting on Stream changes.</p>
  
  <pre><code>body: StreamBuilder(
          stream: someStream,
          builder: (ctx, snapshot) {
            return ListView.separated(
              separatorBuilder: (context, index) =&gt; Divider(color: Colors.black),
              itemCount: ***whatsHere***?,
              itemBuilder: (BuildContext ctx, int index) {
  ...
  </code></pre>
  
  <p>Hope I've missed something.
  Since an idea to get somehow the length of the source stream looks at least strange, because of the asynchronous nature of the streams.
  It seems might be feasible by StatefullWidget with stream subsriptions (and setState invoking), 
  but StreamBuilder is invented to do exactly the same, isn't it?</p>