The gist: ASPNETCORE_ENVIRONMENT variables map directly to the appsettings.json files provided with the template by name. Such that appsettings.Development.json references the environment variable with the value “Development”.

In order to target some framework packages in Visual Studio 2017 RC, you should ensure that you have the latest Windows Insider Preview release installed. This will require that you participate in the Windows Insider Program, which is Microsoft's platform for distributing pre-release versions of Windows 10 and the accompanying .NET Framework SDK's.