When trying to build the project to Windows Store Apps platform, it produces this error:

ArgumentException: The Assembly Microsoft.CSharp is referenced by Facebook.Client ('Assets/Plugins/WSA/Facebook.Client.dll'). But the dll is not allowed to be included or could not be found.
UnityEditor.AssemblyHelper.AddReferencedAssembliesRecurse (System.String assemblyPath, System.Collections.Generic.List`1 alreadyFoundAssemblies, System.String[] allAssemblyPaths, System.String[] foldersToSearch, System.Collections.Generic.Dictionary`2 cache, BuildTarget target) (at C:/buildslave/unity/build/Editor/Mono/AssemblyHelper.cs:160)
UnityEditor.AssemblyHelper.FindAssembliesReferencedBy (System.String[] paths, System.String[] foldersToSearch, BuildTarget target) (at C:/buildslave/unity/build/Editor/Mono/AssemblyHelper.cs:198)
UnityEditor.HostView:OnGUI()

We have other plugin dll's built using the same way as how we built that Facebook.Client.dll, and it works fine.

Repro steps:
1. Open project "case_677882-cleared-TestDLL.zip";
2. Build for WSA, select Universal 8.1 SDK (or any SDK will produce the same error);
3. Observe that the building process has failed;

You can also try remove the Facebook.Client.dll and rebuild the project, the build will success.

Reproducible on 5.0 versions;