title: Access of global variables in dart / flutter
body: <p>I can't seem to access my global variables <em>_firestore</em> and <em>loggedInUser</em> from a different class in flutter / dart. From what I read in the documentation and online, this should be possible?</p>
  <p><strong>Creating the global variables:</strong></p>
  <pre><code>   import 'package:cloud_firestore/cloud_firestore.dart';
      
      final _firestore = Firestore.instance;
      FirebaseUser loggedInUser;
      
      class TheSlateScreen extends StatefulWidget {
  </code></pre>
  <p><strong>Trying to access them here:</strong></p>
  <pre><code>import 'package:theslate/screens/theslate_screen.dart';
  
  class SlateTasks extends ChangeNotifier {
  
   void addTask(String newTaskTitle) {
  
      _firestore.collection('Test').add({
        'Task': newTaskTitle,
        'User': loggedInUser.email,
      });
  </code></pre>