React to HtmlString removal from Mvc.

aspnet/Mvc#4558

Author: N. Taylor Mullen

src/Microsoft.AspNetCore.AngularServices/PrimeCacheHelper.cs

@@ -2,6 +2,7 @@
 using System.Net;
 using System.Net.Http;
 using System.Threading.Tasks;
+using Microsoft.AspNetCore.Html;
 using Microsoft.AspNetCore.Mvc.Rendering;
 using Microsoft.Extensions.Logging;
 using Newtonsoft.Json;