function withAsync ($validator, $watchTargets = []) {
  const validatorObj = normalizeValidatorObject($validator)
  return {
    ...validatorObj,
    $async: true,
    $watchTargets
  }
}
Wraps validators that returns a Promise.
@param {asyncValidator} $validator
@param {watchTargets} $watchTargets
@return {{$async: boolean, $validator: asyncValidator, $watchTargets: watchTargets}}