https://github.com/magento/magento2/blob/2.0.0/lib/internal/Magento/Framework/Data/Form/Element/AbstractElement.php#L406-L414
https://github.com/magento/magento2/blob/2.0.0/lib/internal/Magento/Framework/Data/Form/Element/AbstractElement.php#L371-L373
https://github.com/magento/magento2/blob/2.0.0/lib/internal/Magento/Framework/Data/Form/Element/Fieldset.php#L38-L55
https://github.com/magento/magento2/issues/2952