in lib/src/hierarchy_view.dart [267:276]
TableCellElement _verticalCell(dynamic upper, dynamic lower,
{String align: 'left', String colspan: '1'}) {
DivElement div = new DivElement();
div.children.addAll([
upper is SpanElement ? upper : _span(upper),
new BRElement(),
lower is SpanElement ? lower : _span(lower)
]);
return _cell(div, align: align, colspan: colspan, pre: false);
}