add The searchTerms RxJS subject

src/app/hero-search/hero-search.component.ts

@@ -37,4 +37,12 @@ export class HeroSearchComponent implements OnInit {
       switchMap((term: string) => this.heroService.searchHeroes(term)),
     );
   }
+
+  private searchTerms = new Subject<string>();
+
+  // Push a search term into the observable stream.
+  search(term: string): void {
+    this.searchTerms.next(term);
+  }
+
 }