feat(router): add a provider making angular1/angular2 integration easier (#12769)

modules/@angular/router/src/router_module.ts

@@ -278,11 +278,20 @@ export function initialRouterNavigation(
   };
 }
 
+/**
+ * A token for the router initializer that will be called after the app is bootstrapped.
+ *
+ * @experimental
+ */
+export const ROUTER_INITIALIZER = new OpaqueToken('Router Initializer');
+
 export function provideRouterInitializer() {
-  return {
-    provide: APP_BOOTSTRAP_LISTENER,
-    multi: true,
-    useFactory: initialRouterNavigation,
-    deps: [Router, ApplicationRef, RouterPreloader, ROUTER_CONFIGURATION]
-  };
+  return [
+    {
+      provide: ROUTER_INITIALIZER,
+      useFactory: initialRouterNavigation,
+      deps: [Router, ApplicationRef, RouterPreloader, ROUTER_CONFIGURATION]
+    },
+    {provide: APP_BOOTSTRAP_LISTENER, multi: true, useExisting: ROUTER_INITIALIZER}
+  ];
 }