fix(router-deprecated): export RootRoute

Fix #8449

Closes #8450

modules/@angular/router-deprecated/router.ts

@@ -4,7 +4,7 @@
  * Maps application URLs into application states, to support deep-linking and navigation.
  */
 
-export {Router} from './src/router';
+export {Router, RootRouter} from './src/router';
 export {RouterOutlet} from './src/directives/router_outlet';
 export {RouterLink} from './src/directives/router_link';
 export {RouteParams, RouteData} from './src/instruction';