title: React-Navigation: Route should declare a screen... error
body: <p>I am new to react-navigation and I followed the steps on the <a href="https://reactnavigation.org/docs/intro/" rel="nofollow noreferrer">site</a>, however I get an error saying Route 'Chat' should declare a screen... Below is my code for reference. </p>
  
  <pre><code>import React from 'react';
  import {
    AppRegistry,
    Text,
    View,
    Button,
  } from 'react-native';
  import { StackNavigator } from 'react-navigation';
  
  class HomeScreen extends React.Component {
    static navigationOptions = {
      title: 'Welcome',
    };
    render() {
      const { navigate } = this.props.navigation;
      return (
        &lt;View&gt;
          &lt;Text&gt;Hello, Chat App!&lt;/Text&gt;
          &lt;Button
            onPress={() =&gt; navigate('Chat')}
            title="Chat with Lucy"
          /&gt;
        &lt;/View&gt;
      );
    }
  }
  
  AppRegistry.registerComponent('navigationApp', () =&gt; navigationApp);
  </code></pre>
  
  <p>This is where I believe the error is occurring</p>
  
  <pre><code>const navigationApp = StackNavigator({
    Home: { screen: HomeScreen },
    Chat: { screen: ChatScreen },
  });
  
  class ChatScreen extends React.Component {
    static navigationOptions = {
      title: 'Chat with Lucy',
    };
    render() {
      return (
        &lt;View&gt;
          &lt;Text&gt;Chat with Lucy&lt;/Text&gt;
        &lt;/View&gt;
      );
    }
  }
  </code></pre>