title: popToRoute() not working when I pass in route object
body: <p>I've googled this a bunch and there aren't sufficient examples or explanations that tell me how popToRoute(), the react-native navigator method exactly works. Would be awesome if someone could help me better understand it.</p>
  
  <p>I'm passing in my route object (which is definitely correct) but get the error: "Invariant Violation: Calling popToRoute for a route that doesn't exist!"</p>
  
  <p>Here is the relevant code where I use the popToRoute() navigator method:</p>
  
  <pre><code>var InviteRoommates = React.createClass({
    submitRoommates: function() {
      this.props.navigator.popToRoute({
        index: 3,
        id: 'ItemList'
      });
    },
    render: function() {
      return (
        &lt;View style={styles.container}&gt;
          &lt;Text&gt;INVITE ROOMMATES&lt;/Text&gt;
          &lt;TextInput style={styles.input} placeholder="roommate's email address"/&gt;
          &lt;TouchableHighlight
            style={styles.button}
            onPress={() =&gt; this.submitRoommates()}
          &gt;
            &lt;Text style={styles.btnText}&gt;Submit&lt;/Text&gt;
          &lt;/TouchableHighlight&gt;
        &lt;/View&gt;
      );
    }
  });
  </code></pre>