title: Could not load file or assembly 'System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' or one of its dependencies
body: <p>I'm using Xamarin.Forms in Visual Studio 2015 to develop a cross-platform app for mobile. When adding references .dll files, android app is running ok, but app for windows phone 8.1 it has an error:</p>
  
  <pre><code>Exception thrown: 'System.IO.FileNotFoundException' in MyApp.ni.DLL
  Additional information: Could not load file or assembly 'System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' or one of its dependencies. The system cannot find the file specified.
  </code></pre>
  
  <p>Is there any way to solve this problem?</p>
  
  <p>Specifications:
  Microsoft Visual Studio Community 2015
  Xamarin 4.1.2.18
  Xamarin.Forms 2.3.0.107
  Targets:
  .NET Framework 4.5
  ASP.NET Core 1.0
  Windows 8
  Windows Phone 8.1
  XamarinAndroid
  Xamarin.iOS
  Xamarin.iOS (Classic) </p>
  
  <p>Tested apps on Samsung Note 4 (Marshmallow API-23) and Microsoft Lumia 640 XL (Windows 10 Mobile 10.0.10586.494).</p>