Create a new shared package, Microsoft.AspNet.SpaServices, to hold MapSpaFallbackRoute (and other common infrastructure yet to be added)

Microsoft.AspNet.SpaServices/SpaRouteExtensions.cs

@@ -0,0 +1,53 @@
+using System;
+using System.Collections.Generic;
+using Microsoft.AspNet.Routing;
+using Microsoft.AspNet.SpaServices;
+
+// Putting in this namespace so it's always available whenever MapRoute is
+namespace Microsoft.AspNet.Builder
+{
+    public static class SpaRouteExtensions
+    {
+        private const string ClientRouteTokenName = "clientRoute";
+
+        public static void MapSpaFallbackRoute(this IRouteBuilder routeBuilder, string name, object defaults, object constraints = null, object dataTokens = null)
+        {
+            MapSpaFallbackRoute(routeBuilder, name, /* templatePrefix */ (string)null, defaults, constraints, dataTokens);
+        }
+
+        public static void MapSpaFallbackRoute(this IRouteBuilder routeBuilder, string name, string templatePrefix, object defaults, object constraints = null, object dataTokens = null)
+        {
+            var template = CreateRouteTemplate(templatePrefix);
+
+            var constraintsDict = ObjectToDictionary(constraints);
+            constraintsDict.Add(ClientRouteTokenName, new SpaRouteConstraint(ClientRouteTokenName));
+
+            routeBuilder.MapRoute(name, template, defaults, constraintsDict, dataTokens);
+        }
+
+        private static string CreateRouteTemplate(string templatePrefix)
+        {
+            templatePrefix = templatePrefix ?? string.Empty;
+
+            if (templatePrefix.Contains("?")) {
+                // TODO: Consider supporting this. The {*clientRoute} part should be added immediately before the '?'
+                throw new ArgumentException("SPA fallback route templates don't support querystrings");
+            }
+            
+            if (templatePrefix.Contains("#")) {
+                throw new ArgumentException("SPA fallback route templates should not include # characters. The hash part of a URI does not get sent to the server.");
+            }
+            
+            if (templatePrefix != string.Empty && !templatePrefix.EndsWith("/")) {
+                templatePrefix += "/";
+            }
+
+            return templatePrefix + $"{{*{ ClientRouteTokenName }}}";
+        }
+        
+        private static IDictionary<string, object> ObjectToDictionary(object value)
+        {
+            return value as IDictionary<string, object> ?? new RouteValueDictionary(value);
+        }
+    }
+}