title: React Native - async/await does not work
body: <p>Is it possible to use async/await outside of classes?
  For example I use AsyncStorage to store access token and want to get this token before StackNavigator will be initialized.</p>
  
  <p><strong>container.js</strong></p>
  
  <pre><code>import React from 'react';
  import { StackNavigator } from 'react-navigation';
  import PairingView from '../components/PairingView';
  import MainView from '../components/MainView';
  import { getTokenFromStorageAsync } from '../helpers/asyncStorageHelper';
  
  const accessToken = getTokenFromStorageAsync().done();
  console.log(accessToken);   &lt;---------- undefined
  
  const AppNavigator = StackNavigator({
    PairingRoute: {
      screen: PairingView
    },
    MainRoute: {
      screen: MainView
    }
  }, {
    initialRouteName: (accessToken == null) ? 'PairingRoute' : 'MainRoute',
    initialRouteParams: {
      token: accessToken
    }
  });
  
  const App = () =&gt; (
    &lt;AppNavigator /&gt;
  );
  
  export default App;
  </code></pre>
  
  <p><strong>asyncStorageHelper.js</strong></p>
  
  <pre><code>import { AsyncStorage } from 'react-native';
  
  export const getTokenFromStorageAsync = async () =&gt; {
    try {
      const value = await AsyncStorage.getItem('@nfs:token');
      console.log(value);   &lt;---------- access token
      if (value != null)
        return value;
    } catch (err) {
      console.error(err);
    }
    return undefined;
  };
  </code></pre>