The MSDN documentation for the classes [1] clearly states the supported platforms. Admittedly the restriction to store apps is missing on the page for the namespace [2].

  Minimum supported client  Windows 8.1 [Windows Store apps only]
  Minimum supported server  Windows Server 2012 R2 [Windows Store apps only]
  Minimum supported phone   Windows Phone 8

[1] http://msdn.microsoft.com/en-us/library/windows/apps/xaml/wi...

[2] http://msdn.microsoft.com/en-us/library/windows/apps/xaml/wi...