message: docs(forms): add an example for async validator token
  to be consistent with the validator token, have the example code for the async validator token.
  PR Close
diff: @@ -60,6 +60,26 @@ export const NG_VALIDATORS = new InjectionToken<Array<Validator|Function>>('NgVa
  *
  * @see `NG_VALIDATORS`
  *
  + * @usageNotes
  + *
  + * ### Provide a custom async validator directive
  + *
  + * The following example implements the `AsyncValidator` interface to create an
  + * async validator directive with a custom error key.
  + *
  + * ```typescript
  + * @Directive({
  + * selector: '[customAsyncValidator]',
  + * providers: [{provide: NG_ASYNC_VALIDATORS, useExisting: CustomAsyncValidatorDirective, multi:
  + * true}]
  + * })
  + * class CustomAsyncValidatorDirective implements AsyncValidator {
  + * validate(control: AbstractControl): Promise<ValidationErrors|null> {
  + * return Promise.resolve({'custom': true});
  + * }
  + * }
  + * ```
  + *
  * @publicApi
  */
  export const NG_ASYNC_VALIDATORS =
  