title: add an item to ListView.builder flutter firebase
body: <p>I'm trying to add a header item at the beginning of my list, so here is how I build it using firebase</p>
  
  <pre><code>new Expanded(
              child: new StreamBuilder(
                  stream: Firestore.instance
                      .collection("users")
                      .document("dana")
                      .collection("Channels")
                      .snapshots(),
                  builder: (context, snapshot) {
                    return new ListView.builder(
                      scrollDirection: Axis.vertical,
                      itemCount: snapshot.data.documents.length,
                      itemBuilder: (context, index) =&gt; _buildList(
                          context, snapshot.data.documents[index]),
                    );
                  }),
            )
  </code></pre>
  
  <p>"_buildList" is just the Widget</p>
  
  <pre><code>Widget _buildListItem(BuildContext context, DocumentSnapshot document) 
  </code></pre>
  
  <p>so I'm basically lost, I've no idea how to add another Widget as header
  any suggestions?</p>