public sealed class RestConfigurationSource : BaseConfigurationSource
    {
//...
//HttpClient is from:
// Assembly netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51
// C:\Program Files\dotnet\sdk\NuGetFallbackFolder\netstandard.library\2.0.3\build\netstandard2.0\ref\netstandard.dll
        internal RestConfigurationSource(HttpClient httpClient)
        {
//...

        internal static RestConfigurationSource CreateSource()
        {
            FakeHttpMessageHandler fakeResponseHandler = CreateFakeResponseHandler();
//HttpClient is from:
//System.Net.Http, Version=4.2.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
//C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\MSBuild\Microsoft\Microsoft.NET.Build.Extensions\net461\lib\System.Net.Http.dll
            var client = new HttpClient(fakeResponseHandler);
//...
            var configSrc = new RestConfigurationSource(client);

using System.Net.Http;

namespace ClassLibrary1.STD {
    public class Class1Std {
        public Class1Std(HttpClient client) {
        }
    }
}

using ClassLibrary1.STD;
using System.Net.Http;

namespace ClassLibrary1.Fx {
    public class Class1Fx : Class1Std {
        public Class1Fx(HttpClient client)
            : base(client) {
        }
    }
}

    [TestClass]
    public class UnitTest1 {
        [TestMethod]
        public void TestMethod1() {
            var c1 = CreateClass();
        }

        private Class1Fx CreateClass() {
            HttpClient client = new HttpClient();
            Class1Fx c1 = new Class1Fx(client);
            return c1;
        }
    }

    class Program {
        static void Main(string[] args) {
            var c1 = CreateClass();
        }

        private static Class1Fx CreateClass() {
            HttpClient client = new HttpClient();
            Class1Fx c1 = new Class1Fx(client);
            return c1;
        }
    }