title: Class 'Future<dynamic>' has no instance method
body: <p>I have some code to get files from a json file, only I get a message and not know how to solve that, any ideas?:</p>
  <pre><code>List&lt;LatLng&gt; polylineCoordinates = [];
  
  @override
    void initState() {
      polylineCoordinates = readJson().map((e)=&gt; LatLng(e['lat'], e['long'])).tolist();
      super.initState();
    }
  </code></pre>
  <p>The error is:</p>
  <pre><code>Class 'Future&lt;dynamic&gt;' has no instance method 'map'.
  Receiver: Instance of 'Future&lt;dynamic&gt;'
  Tried calling: map(Closure: (dynamic) =&gt; LatLng)
  </code></pre>