title: How to react to firebase auth email change
body: <p>I'd like a safe way to update a user document in Firestore when a user changes their email address.</p>
  
  <p>At the moment, the only way I can see to do it is by doing it client-side, after the <code>user.updateEmail()</code> is successfully called, but that call could fail for a variety of reasons, resulting in a mismatch between emails on the auth &amp; the user's doc.</p>
  
  <p>I was expecting to find a firebase function like <code>functions.auth().onChange()</code> but nothing like that seems to exist.</p>