title: Angular 2 or 4 Reactive form set validation depend on another field value
body: <p>I'm trying to build out a form in Angular 2 using the Reactive Forms Module, and I'm trying to set validation of some field in a form that's required if another field is select.<br>
  Have I done something wrong?</p>
  
  <pre><code>ngOnInit() {
      this.martialForm = this.fb.group({
          'maritalStatus': [this.model.maritalStatus, [Validators.required]],
          'spouseTitle': [this.spouseModel.title, null]
      },{
           validator: this.validateIsSpouse
      })
  }
  
  validateIsSpouse(group: FormGroup) {
      if(this.model.maritalStatus == "01"){
          group.controls['spouseTitle'].setErrors({ isRequired: true });
      }
  
      return null;
  }
  </code></pre>