无法使用Dart和angular2客户端运行GolLang后端

I use this tutorial to make a web application with GoLang, Angular2 and Dart, but when i start backend by console command 'backend', and route in browser to "localhost:8080/" it must call method from Dart's class "Hello" but it doesn't call, and i get 404 error. All code i got from tutorial and didn't change anything. And i can't find any other tutorials. Can you explain me what wrong am i doing?

GoLang code:

func main() {
   http.Handle("/", http.FileServer(http.Dir("./app/web/")))

   fmt.Println("Text")
   http.HandleFunc("/api/hello", helloWorld)
   http.ListenAndServe(":8080", nil)

}
func helloWorld(w http.ResponseWriter, r *http.Request) {
   data := struct {
       Message string
   }{
       "Hello, World",
   }

   if err := json.NewEncoder(w).Encode(data); err != nil {
       log.Println(err)
   }
}

and angular dart code:

class AppComponent {
   Hello hello = new Hello();
}

class Hello{
  String message;

  Hello(){

    HttpRequest.getString('/api/hello')
        .then((String content) {
          Map parsedMap = JSON.decode(content);
          message = parsedMap["Message"];
        })
        .catchError((Error error) {
          print(error.toString());
        });
  }
}

and project structure :