Cómo crear un validador en Angular 2

<!DOCTYPE html>
<html>

  <head>
    <base href="." />
    <title>angular2 playground</title>
    <link rel="stylesheet" href="style.css" />
    <link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">
    <script src="https://unpkg.com/zone.js/dist/zone.js"></script>
    <script src="https://unpkg.com/zone.js/dist/long-stack-trace-zone.js"></script>
    <script src="https://unpkg.com/reflect-metadata@0.1.3/Reflect.js"></script>
    <script src="https://unpkg.com/systemjs@0.19.31/dist/system.js"></script>
    <script src="config.js"></script>
    <script>
    System.import('app')
      .catch(console.error.bind(console));
  </script>
  </head>

  <body>
    <my-app>
    loading...
  </my-app>
  </body>

</html>

/* Styles go here */

### Angular Starter Plunker - Typescript

System.config({
  //use typescript for compilation
  transpiler: 'typescript',
  //typescript compiler options
  typescriptOptions: {
    emitDecoratorMetadata: true
  },
  paths: {
    'npm:': 'https://unpkg.com/'
  },
  //map tells the System loader where to look for things
  map: {
    
    'app': './src',
    
    '@angular/core': 'npm:@angular/core/bundles/core.umd.js',
    '@angular/common': 'npm:@angular/common/bundles/common.umd.js',
    '@angular/compiler': 'npm:@angular/compiler/bundles/compiler.umd.js',
    '@angular/platform-browser': 'npm:@angular/platform-browser/bundles/platform-browser.umd.js',
    '@angular/platform-browser-dynamic': 'npm:@angular/platform-browser-dynamic/bundles/platform-browser-dynamic.umd.js',
    '@angular/http': 'npm:@angular/http/bundles/http.umd.js',
    '@angular/router': 'npm:@angular/router/bundles/router.umd.js',
    '@angular/forms': 'npm:@angular/forms/bundles/forms.umd.js',
    
    '@angular/core/testing': 'npm:@angular/core/bundles/core-testing.umd.js',
    '@angular/common/testing': 'npm:@angular/common/bundles/common-testing.umd.js',
    '@angular/compiler/testing': 'npm:@angular/compiler/bundles/compiler-testing.umd.js',
    '@angular/platform-browser/testing': 'npm:@angular/platform-browser/bundles/platform-browser-testing.umd.js',
    '@angular/platform-browser-dynamic/testing': 'npm:@angular/platform-browser-dynamic/bundles/platform-browser-dynamic-testing.umd.js',
    '@angular/http/testing': 'npm:@angular/http/bundles/http-testing.umd.js',
    '@angular/router/testing': 'npm:@angular/router/bundles/router-testing.umd.js',
    
    'rxjs': 'npm:rxjs',
    'typescript': 'npm:typescript@2.0.2/lib/typescript.js'
  },
  //packages defines our app package
  packages: {
    app: {
      main: './main.ts',
      defaultExtension: 'ts'
    },
    rxjs: {
      defaultExtension: 'js'
    }
  }
});

//main entry point
import {platformBrowserDynamic} from '@angular/platform-browser-dynamic';
import {AppModule} from './app';

platformBrowserDynamic().bootstrapModule(AppModule)

//our root app component
import {Component, NgModule} from '@angular/core'
import { FormsModule } from '@angular/forms';
import {BrowserModule} from '@angular/platform-browser'

import { HouseStarkValidator } from './validators/house-stark-validator.directive';

@Component({
  selector: 'my-app',
  template: `
    <div class="container">
      <form name="demoForm">
        <h1>House Stark Form</h1>
        <div class="form-group" [ngClass]="{'has-success': character.dirty && character.valid, 'has-error': character.dirty && character.invalid}">
          <label class="control-label" for="character">Type the name of your favourite Stark character</label>
          <input class="form-control" type="text" name="character" [(ngModel)]="model.character" stark #character="ngModel" placeholder="Eddard">
          <span *ngIf="!character.errors?.stark" class="help-block">Hint: Jon Snow is not a rightful Stark</span>
          <span *ngIf="character.errors?.stark" class="help-block">Oops! Not a legitimate Stark!</span>
        </div>
      </form>
    </div>
  `,
})
export class App {
  model = {};
}

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

import { Directive, forwardRef } from '@angular/core';
import { NG_VALIDATORS, Validator } from '@angular/forms';

const HOUSE_STARK_VALIDATOR: any = {
  provide: NG_VALIDATORS,
  useExisting: forwardRef(() => HouseStarkValidator),
  multi: true
};

@Directive({
  selector: '[stark]',
  providers: [ HOUSE_STARK_VALIDATOR ]
})
export class HouseStarkValidator implements Validator {
  private LEGITIMATE_STARK_MEMBERS: Array<string> = [
    'EDDARD', 
    'BENJEN', 
    'LYANNA', 
    'CATELYN', 
    'ROBB', 
    'SANSA', 
    'ARYA', 
    'BRANDON', 
    'RYCKON'
  ];

  validate(control: AbstractControl) : {[key: string]: any} {
    if (control.value != null || typeof control.value === 'string' && control.value.length !== 0) {
      return this.isLegitimateStark(control.value) ? null : { 'stark': true };
    } else {
      return null;
    }
  }

  private isLegitimateStark (name: string): boolean {
    return this.LEGITIMATE_STARK_MEMBERS.indexOf(name.toUpperCase().trim()) !== -1;
  }
}