title: onUpdate() never triggers
body: <p>I'm writing a simple node app using firebase. It has a list of tasks that can be checked off by the user, which updates the database. I'm trying listen for changes on the database and update each checkbox when the boolean associated with it on the database is changed. However, the onUpdate never triggers.<br><br> My database looks like this:<br></p>
  
  <pre><code>tasks
  ├── 0
  |   ├── text: "task 1"
  |   └── isDone: bool
  ├── 1
  |   ├── text: "task 2"
  |   └── isDone: bool
  etc...
  </code></pre>
  
  <p><br>
  Here's my code for onUpdate:</p>
  
  <pre><code>const ref = functions.database.ref('tasks')
  .onUpdate((snap, context) =&gt; {
      console.log("Updated!");
      snap.forEach(shot =&gt; {
          el = document.getElementById(shot.toString());
          el.checked = shot.isDone;
      });
  })
  </code></pre>
  
  <p>(The ids of my checkboxes are numbered from 0)</p>