title: Angular Fire: undefined is not a function ($on method does not exist)
body: <p>I'm trying to log my Firebase data to the console. But I keep getting an error of <code>undefined is not a function</code>. Full error below:</p>
  
  <pre><code>TypeError: undefined is not a function
  at Object.childAdded (http://localhost:9000/scripts/services/ProviderService.js:11:22)
  at new &lt;anonymous&gt; (http://localhost:9000/scripts/controllers/main.js:118:19)
  </code></pre>
  
  <p>This is what my code currently looks like:</p>
  
  <p><strong>ProviderService.js</strong></p>
  
  <pre><code>angular.module('outcomesApp').service('ProviderService', function(FBURL, $q, $firebase) {
    var providerRef = new Firebase(FBURL).child('providers');
    var fireProvider = $firebase(providerRef).$asArray();
  
    return {
      childAdded: function childAdded(cb) {
        fireProvider.$on('child_added', function(data) {
          console.log(data);
        });
      },
      ...
  </code></pre>
  
  <p><strong>main.js</strong></p>
  
  <pre><code>app.controller('MainCtrl', function ($scope, $filter, $timeout, $firebase, FBURL, ProviderService) {
    ProviderService.childAdded(function(addedChild) {
      $scope.providers.push(addedChild);
    });
    ...
  </code></pre>
  
  <p>The error is occurring on this line: <code>fireProvider.$on('child_added', function(data) {</code></p>