title: AddAsync() method is missing
body: <p>I am using Entity Framework Core. I want to create an async method which will create new user in my database, i have included all the libraries I need, but some methods that are supposed to work with database are missing, I have almost every async method but i am missing AddAsync and RemoveAsync. When I type AddAsync manually I get the following error message: "Error   CS1061  'DbSet' does not contain a definition for 'AddAsync' and no accessible extension method 'AddAsync' accepting a first argument of type 'DbSet' could be found (are you missing a using directive or an assembly reference?)"</p>
  
  <p>The class where the method is created has following code and libraries included:</p>
  
  <pre><code>using System;
  using System.Collections.Generic;
  using System.Linq;
  using System.Text;
  using System.Threading.Tasks;
  using System.Data.Entity;
  using System.Data;
  using Microsoft.EntityFrameworkCore;
  namespace ClassLibrary
  {
      public class Class1 : Interface1
      {
          public async Task AddKorisnik(Korisnici k)
          {
              using (ExtentEntities context = new ExtentEntities())
              {
                  context.Korisnici.AddAsync();
                  await context.SaveChangesAsync();
              }
          }
      }
  }
  
  </code></pre>
  
  <p>The class where the DbContext is used is following:</p>
  
  <pre><code>namespace ClassLibrary
  {
      using System;
      using System.Linq;
      using System.Data.Entity;
      using System.Data.Entity.Infrastructure;
  
      public partial class ExtentEntities : DbContext
      {
          public ExtentEntities()
              : base("name=ExtentEntities")
          {
          }
  
          protected override void OnModelCreating(DbModelBuilder modelBuilder)
          {
              throw new UnintentionalCodeFirstException();
          }
  
          public virtual DbSet&lt;Korisnici&gt; Korisnici { get; set; }
      }
  }
  </code></pre>