xamarin-forms-samples/UserInterface/Layout/RelativeLayoutDemos/RelativeLayoutDemos.UWP/App.xaml

<Application
    x:Class="RelativeLayoutDemos.UWP.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:RelativeLayoutDemos.UWP">

</Application>