title: FieldValue is undefined
body: <p>I'm using firestore in my react app. It is working without any problems when trying to get/create/update documents.
  However I am unable to access FieldValue which I need to update an array using arrayUnion. My firebase version used is 6.6.2.</p>
  
  <p>Specifically this is the code I'm trying to get to work:</p>
  
  <pre><code>const locations = firebase.firestore.FieldValue.arrayUnion('location 1')
  firebase.firestore().collection('config').doc('groups').update({ locations })
  </code></pre>
  
  <p><code>firebase.firestore()</code> is working in the exact same file but <code>firebase.firestore.FieldValue</code> is undefined.</p>