title: FieldValue undefined when using functions and Firestore
body: <p>I have the following function:</p>
  
  <pre><code>const functions = require('firebase-functions');
  const admin = require('firebase-admin');
  const FieldValue = require('firebase-admin').FieldValue;
  
  module.exports = functions.firestore
    .document('students/{studentId}')
    .onDelete(event =&gt; {
      const student = event.data.previous.data();
      const { id, semester } = student;
      const classId = student.class;
      const deleteObj = {};
      deleteObj[id] = FieldValue.delete(); //Line 12, this is where the error orccurs
      return admin
        .firestore()
        .collection('semesters')
        .doc(semester)
        .collection('students')
        .doc(classId)
        .update(deleteObj);
    });
  </code></pre>
  
  <p>Every time i run it I get the following error:</p>
  
  <pre><code>TypeError: Cannot read property 'delete' of undefined
      at module.exports.functions.firestore.document.onDelete.event (/user_code/deleteStudent.js:12:37)
  </code></pre>
  
  <p>Looking at the <a href="https://firebase.google.com/docs/firestore/manage-data/delete-data#fields" rel="noreferrer">docs</a> I cant really see what I am doing wrong?</p>
  
  <pre><code>// Get the `FieldValue` object
  var FieldValue = require("firebase-admin").FieldValue;
  
  // Create a document reference
  var cityRef = db.collection('cities').doc('BJ');
  
  // Remove the 'capital' field from the document
  var removeCapital = cityRef.update({
      capital: FieldValue.delete()
  });
  </code></pre>
  
  <p><strong>Update</strong></p>
  
  <p>So, using the web equivalent seems to work: <code>admin.firestore.FieldValue.delete()</code>. But that seems like a bug since i'm in a nodejs environment? Can any Firebaser confirm or deny wether that is the case or not? I'd gladly file a bug report.</p>