title: Error while adding validator to reactive form
body: <p>I have created a simple form to which I would like to add a validator:</p>
  
  <pre><code> form;
  
    constructor() { 
      this.form = new FormGroup({
      'old-password' : new FormControl('', [Validators.required, Validators.minLength(4)], PasswordValidators.checkPassword),
      'new-password' : new FormControl('', [Validators.required, Validators.minLength(4)]),
      'confirm-password' : new FormControl('', [Validators.required, Validators.minLength(4)])
    }, {
        validator: PasswordValidators.passwordsShouldMatch
      });
    }
  </code></pre>
  
  <p>But in <code>validator</code> section I get an error which says:</p>
  
  <pre><code>[ts]
  Argument of type '{ validator: (control: AbstractControl) =&gt; { passwordsShouldMatch: boolean; }; }' is not assignable to parameter of type 'ValidatorFn'.
    Object literal may only specify known properties, and 'validator' does not exist in type 'ValidatorFn'.
  (property) validator: (control: AbstractControl) =&gt; {
      passwordsShouldMatch: boolean;
  }
  </code></pre>
  
  <p>When I change the construction of the form to <code>FormBuilder</code> it starts to working - why's that?</p>