Propose to specify the $element parameter class name in the signature of the \Magento\Config\Block\System\Config\Form\Field::_decorateRowHtml() method

https://github.com/magento/magento2/blob/2.0.0/app/code/Magento/Config/Block/System/Config/Form/Field.php#L177-L187

Propose to change it to:

protected function _decorateRowHtml(
    \Magento\Framework\Data\Form\Element\AbstractElement $element, $html
)

https://github.com/magento/magento2/issues/2819