title: IServiceCollection does not contain a defintion for AddHttpClient
body: <p>I am trying to use HttpClient in my .net core 2.0 project and for that I have injected HttpClient in my controller. But when I am trying to configure httpclient in my startup.cs I am getting this error: "IServiceCollection does not contain a defintion for AddHttpClient". I have already referenced <code>using Microsoft.AspNetCore.Http;</code> and <code>using Microsoft.Extensions.DependencyInjection;</code> and here is what I am trying to do: </p>
  
  <pre><code>services.AddHttpClient&lt;TestController&gt;();
  </code></pre>
  
  <p>It's working fine in other project with same namespaces but here I am getting the error. Any help?</p>