Replace dependency (NuGet package ref) with frameworkAssembly

[MatthewSteeples]

No description provided.

[clairernovotny]

Thanks for the PR. Unfortunately, as-is, this will not work as it breaks support for .NET Core.

We do not need frameworkAssembly here, we really do need the dependencies. Please see my blog post for the detailed explanation:
https://oren.codes/2015/07/29/targeting-net-core/

These dependencies do install on .NET 4.5 and they're harmless. Trying to add empty dependency groups will cause issues down the road as specific TFM > dotnet > portable-*. These packages are the same reference assemblies that live on your computer used by the PCL project system but are now available to dotnet CLI and project.json.

[clairernovotny]

closing as this should be resolved in 2.0.1