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

Source 

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