title: Page navigation WP8.1
body: <p>I made an app to WP8 and I used the page navigation like <code>NavigationService.Navigate(new Uri(...))</code> and worked well.<br>
  But now I'm trying to develop a WP8.1 app and can't figure out how is it going.<br>
  I get the following error <code>The name 'NavigationService' does not exist in the current context</code>.<br>
  I have two pages and I want to navigate between them.<br>
  How can I do?<br>
  What changed?</p>