Add new Microsoft.AspNetCore.SpaServices.Extensions package to host new runtime functionality needed for updated templates until 2.1 ships

src/Microsoft.AspNetCore.SpaServices.Extensions/SpaDefaultPageMiddleware.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using System;
+
+namespace Microsoft.AspNetCore.SpaServices
+{
+    internal class SpaDefaultPageMiddleware
+    {
+        public static void Attach(ISpaBuilder spaBuilder)
+        {
+            if (spaBuilder == null)
+            {
+                throw new ArgumentNullException(nameof(spaBuilder));
+            }
+
+            var app = spaBuilder.ApplicationBuilder;
+            var options = spaBuilder.Options;
+
+            // Rewrite all requests to the default page
+            app.Use((context, next) =>
+            {
+                context.Request.Path = options.DefaultPage;
+                return next();
+            });
+
+            // Serve it as file from wwwroot (by default), or any other configured file provider
+            app.UseStaticFiles(new StaticFileOptions
+            {
+                FileProvider = options.DefaultPageFileProvider
+            });
+
+            // If the default file didn't get served as a static file (usually because it was not
+            // present on disk), the SPA is definitely not going to work.
+            app.Use((context, next) =>
+            {
+                var message = "The SPA default page middleware could not return the default page " +
+                    $"'{options.DefaultPage}' because it was not found, and no other middleware " +
+                    "handled the request.\n";
+
+                // Try to clarify the common scenario where someone runs an application in
+                // Production environment without first publishing the whole application
+                // or at least building the SPA.
+                var hostEnvironment = (IHostingEnvironment)context.RequestServices.GetService(typeof(IHostingEnvironment));
+                if (hostEnvironment != null && hostEnvironment.IsProduction())
+                {
+                    message += "Your application is running in Production mode, so make sure it has " +
+                        "been published, or that you have built your SPA manually. Alternatively you " +
+                        "may wish to switch to the Development environment.\n";
+                }
+
+                throw new InvalidOperationException(message);
+            });
+        }
+    }
+}