Add UseReactDevelopmentServer() middleware. Factor out common code.

src/Microsoft.AspNetCore.SpaServices.Extensions/Util/TcpPortFinder.cs

@@ -0,0 +1,25 @@
+// 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 System.Net;
+using System.Net.Sockets;
+
+namespace Microsoft.AspNetCore.SpaServices.Util
+{
+    internal static class TcpPortFinder
+    {
+        public static int FindAvailablePort()
+        {
+            var listener = new TcpListener(IPAddress.Loopback, 0);
+            listener.Start();
+            try
+            {
+                return ((IPEndPoint)listener.LocalEndpoint).Port;
+            }
+            finally
+            {
+                listener.Stop();
+            }
+        }
+    }
+}