title: Angular2 how to unit test a custom validator directive?
body: <p>I wrote a very simple custom validator for an input field:</p>
  <pre class="lang-js prettyprint-override"><code>import { Directive } from '@angular/core';
  import { AbstractControl, NG_VALIDATORS } from '@angular/forms';
  
  function numberValidator(c: AbstractControl) {
      if (!c.value) return null;
      return new RegExp('^[1-9][0-9]{6,9}$').test(c.value) ? null : {
          validateNumber: {
              valid: false
          }
      }
  }
  
  @Directive({
      selector: '[number-validator]',
      providers: [
          { provide: NG_VALIDATORS, multi: true, useValue: numberValidator }
      ]
  })
  export class NumberValidator {}
  </code></pre>
  <p>I would like to unit test this validator. I read <a href="https://angular.io/docs/ts/latest/guide/testing.html#!#attribute-directive" rel="nofollow noreferrer">Test an attribute directive</a> on the Angular2 page, but there is no css or html that changes. How can I unit test this validator?</p>