1. @DomName('HTMLOutputElement.setCustomValidity')
  2. @DocsEditable()
void setCustomValidity(String error)

Source 

@DomName('HTMLOutputElement.setCustomValidity')
@DocsEditable()
void setCustomValidity(String error) => _blink.BlinkHTMLOutputElement.instance
    .setCustomValidity_Callback_1_(this, error);