title: Firestore increment is undefined?
body: <p>I have the following trigger, having a hard time doing an increment on a parent field.
  Any suggestions?</p>
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>const functions = require('firebase-functions');
  const admin = require('firebase-admin');
  const fieldValue = admin.firestore.FieldValue;
  admin.initializeApp();
  
  exports.onTicketCreate = functions.firestore
    .document('/contests/{contestId}/{tickets}/{ticketId}')
    .onWrite((snapshot, context) =&gt; { 
  
      functions.firestore.document('contests/' + context.params.contestId).update({
          points: fieldValue.increment(1)
      });
    });</code></pre>
  </div>
  </div>
  </p>
  <p>Error</p>
  <pre><code>TypeError: Cannot read property 'increment' of undefined
  at exports.onTicketCreate.functions.firestore.document.onWrite (/workspace/lib/index.js:27:48)
  at cloudFunction (/workspace/node_modules/firebase-functions/lib/cloud-functions.js:132:23)
  at Promise.resolve.then (/layers/google.nodejs.functions-framework/functions-framework/node_modules/@google-cloud/functions-framework/build/src/invoker.js:330:28)
  at process._tickCallback (internal/process/next_tick.js:68:7) 
  </code></pre>