Why does Visual Studio tell me that the AddJsonFile() method is not defined?

I'm developing an ASP.NET 5 WebAPI project using VS Ultimate 2015 Preview. I'm trying to configure the app in this way (line numbers are just guides): 1 using Microsoft.Framework.ConfigurationModel; 2 3 public IConfiguration Configuration { get; private set; } 4 5 public Startup() 6 { 7...