title: Streambuilder not updating when firestore changes
body: <p>I have an animated list, the build function is included in a stack from the body of the app.
  Whenever somethings updates in the database, the streambuilder doesn't rebuild..
  items is set in the init by the function:</p>
  
  <pre><code>  Stream&lt;List&lt;Task&gt;&gt; getTasks(){
      try {
        Firestore.instance
            .collection("lists")
            .document(tasklist.postID)
            .collection("tasks")
            .snapshots();
      }
      on Exception {
        error();
      }
      return ref.snapshots().map((list) =&gt;
          list.documents.map((doc) =&gt; Task.fromFireStore(doc)).toList());
  }
  </code></pre>
  
  <pre><code>  Widget _buildTasksList() {
      return new Expanded(
        child: new StreamBuilder(
        stream: items,
        builder: (BuildContext context, AsyncSnapshot snapshot) {
          if (!snapshot.hasData) {
            return new Text("Loading");
          }
          return new AnimatedList(
            initialItemCount: tasks.length,
            key: _listKey,
            itemBuilder: (context, index, animation) {
              print(index);
              return new TaskRow(
                task: listModel[index],
                animation: animation,
                listModel: listModel,
                tasklist: tasklist,
                onChange: () =&gt; _onChange(listModel[index]),
              );
            },
          );
        },
      )
      );
    }
  </code></pre>