import { NgModule }       from '@angular/core';
import { BrowserModule }  from '@angular/platform-browser';
import { FormsModule }    from '@angular/forms'
import { AppComponent }   from './app.component';
@NgModule({
  imports:      [ BrowserModule, FormsModule],
  declarations: [ AppComponent ],
  bootstrap:    [ AppComponent ]
})
export class AppModule { }

import { NgModule }       from '@angular/core';
import { BrowserModule }  from '@angular/platform-browser';
import { FormsModule }    from '@angular/forms'
import { AppComponent }   from './app.component';
import { HeroDetailComponent } from './hero-detail.component';

@NgModule({
  imports:      [ BrowserModule, FormsModule],
  declarations: [ AppComponent, HeroDetailComponent ],
  bootstrap:    [ AppComponent ]
})
export class AppModule { }

Add HeroDetailComponent to the NgModule
Add HeroDetailComponent to the NgModule
