I have a dotnet core app that is targetting Microsoft.NETCore.App 1.1.2. I created a test project to test against that project but when building I noticed this warning:
I had similar problem trying to install Entityframework.Core package in a .NET Core 2 Web app. To solve the issue, I've forced installation through Package Manager Console:
Install-Package Microsoft.NETCore.App -Version 2.0.5
(2.0.5 was the most recent version at the time)
I hope it's useful. Peace.