Requiring a checkbox to be checked

Since Angular 2.3.1 you can use Validators#requiredTrue:

Component:

this.formGroup = this.formBuilder.group({
  cb: [false, Validators.requiredTrue]
});

Template:

<form [formGroup]="formGroup">
  <label><input type="checkbox" formControlName="cb"> Accept it</label>
  <div style="color: red; padding-top: 0.2rem" *ngIf="formGroup.hasError('required', 'cb')">
    Required
  </div>
  <hr>
  <div>
    <button type="submit" [disabled]="formGroup.invalid">Submit</button>
  </div>
</form>

STACKBLITZ DEMO