Today I cloned the aspnet/StaticFiles repository from GitHub on Visual Studio 2015 and when I tried to build the solution I received the following error:
Error: FindPackagesById: Microsoft.AspNet.Hosting.Abstractions
The problem was that the solution was using packages not available on the main NuGet repo. I was able to fix this problem by going to Tools > NuGet Package Manager > Package Manager Settings > Package Sources and then un-checking all the sources except AspNetVNext https://www.myget.org/F/aspnetvnext/api/v2