title: .NET Core 3 preview 4: 'AddNewtonsoftJson' is not defined
body: <p>Using .NET Core 3 preview 4, the "API" template for a F# ASP.NET MVC project fails to build. This is without any changes to the template whatsoever.</p>
  
  <p><a href="https://i.stack.imgur.com/cWJ87.png" rel="noreferrer"><img src="https://i.stack.imgur.com/cWJ87.png" alt="&quot;API&quot; template for a F# ASP.NET MVC project"></a></p>
  
  <p>This is the code that fails:</p>
  
  <pre class="lang-fs prettyprint-override"><code>type Startup private () =
      member this.ConfigureServices(services: IServiceCollection) =
          // Add framework services.
          services.AddControllers().AddNewtonsoftJson() |&gt; ignore
  </code></pre>
  
  <p>With error</p>
  
  <blockquote>
    <p>...\Startup.fs(23,35): error FS0039: The field, constructor or member 'AddNewtonsoftJson' is not defined. Maybe you want one of the following:   AddNewtonsoftJsonProtocol</p>
  </blockquote>
  
  <p>It seems that there are <a href="https://github.com/aspnet/AspNetCore/issues/7438" rel="noreferrer">changes coming for this</a> - is it just being worked on and unusable right now?</p>