Agregando el CORS

startup.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.DependencyInjection;

namespace AdvientoCSharpDic2018
{
    public class Startup
    {
        // This method gets called by the runtime. Use this method to add services to the container.
        // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddDbContext<AdvientoContext>(config=>{
                config.UseNpgsql("Server=127.0.0.1; Port=5432; Database=Adviento2018; User Id=postgres; Password= root;");
            });

            //1.Habilitamos el servicio de CORS
            services.AddCors(options=>{
                options.AddPolicy("AllowSpecific",p=>p.WithOrigins("http://localhost:5000"));
            });

            services.AddMvc();
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }       

            app.UseStaticFiles();
            //2. Utilizamos el CORS especificando la política creada.
            app.UseCors("AllowSpecific"); //Muy importante.
            app.UseMvc(config=>{
                config.MapRoute("Default","/{controller}/{action}/{id?}", new{controller="Home", Action ="Index"});
            });        
        }
    }
}