feat(router): export RouterLink and RouterOutlet (#8912)

Makes it possible to individually import RouterLink and RouterOutlet from the router module

modules/@angular/router/src/directives/router_directives.ts

@@ -0,0 +1,25 @@
+import {RouterOutlet} from "./router_outlet";
+import {RouterLink} from "./router_link";
+
+/**
+ * A list of directives. To use the router directives like {@link RouterOutlet} and
+ * {@link RouterLink}, add this to your `directives` array in the {@link View} decorator of your
+ * component.
+ *
+ * ```
+ * import {Component} from '@angular/core';
+ * import {ROUTER_DIRECTIVES, Routes} from '@angular/router';
+ *
+ * @Component({directives: [ROUTER_DIRECTIVES]})
+ * @Routes([
+ *  {...},
+ * ])
+ * class AppCmp {
+ *    // ...
+ * }
+ *
+ * bootstrap(AppCmp);
+ * ```
+ */
+
+export const ROUTER_DIRECTIVES: any[] = /*@ts2dart_const*/[RouterOutlet, RouterLink];