Reproduction steps:
1. Open any project using 5.6.0f3
2. Build to Universal 8.1 with C# projects enabled
3. Assertion failed message is printed out:

Assertion failed on expression: 'sdk != kUniversalSDK81'
UnityEditorInternal.InternalEditorUtility:GetMonoIslandsForPlayer()
MetroAssemblyCSharpCreator:CreateAssemblyCSharp(CSharpProject, String, String, LibraryCollection, WSASDK, CSharpProject[]) (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/MetroAssemblyCSharpCreator.cs:422)
MetroCSharpVisualStudioSolutionCreator:CreateSolutionFileFrom() (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/MetroCSharpVisualStudioSolutionCreator.cs:819)
PostProcessWSA:CreateVisualStudioSolution() (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/PostProcessWSA.cs:329)
PostProcessUniversal81:CreateVisualStudioSolution() (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/PostProcessUniversal81.cs:169)
PostProcessWinRT:Process() (at C:/buildslave/unity/build/PlatformDependent/WinRT/SharedSources/CSharp/PostProcessWinRT.cs:220)
PostProcessUniversal81:Process() (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/PostProcessUniversal81.cs:41)
UnityEditor.HostView:OnGUI()