ControlGroup(Map<String, AbstractControl> controls, [ Map<String, bool> optionals = null, ValidatorFn validator = null, AsyncValidatorFn asyncValidator = null ])

Source 

ControlGroup(this.controls,
    [Map<String, bool> optionals = null,
    ValidatorFn validator = null,
    AsyncValidatorFn asyncValidator = null])
    : super(validator, asyncValidator) {
  this._optionals = optionals ?? {};
  this._initObservables();
  this._setParentForControls();
  this.updateValueAndValidity(onlySelf: true, emitEvent: false);
}