bool checked

If non-null, sets the "hasCheckedState" semantic to true and the "isChecked" semantic to the given value.

Source 

bool get checked => _checked;
void checked=(bool value)

Source 

set checked(bool value) {
  if (checked == value)
    return;
  bool hadValue = checked != null;
  _checked = value;
  markNeedsSemanticsUpdate(onlyChanges: (value != null) == hadValue);
}