Implementation class without browser-specific hacks.

class CaptionPanelImpl {

 factory CaptionPanelImpl.browserDependant() {
   return new CaptionPanelImpl();
 }

 CaptionPanelImpl();

 void setCaption(dart_html.FieldSetElement fieldset, dart_html.Element legend, String caption, [bool asHtml = true]) {
   // TODO(bruce): rewrite to be inlinable
   assert (caption != null);

   if (asHtml) {
     legend.innerHtml = caption;
   } else {
     legend.text = caption;
   }

   if ("" != caption) {
     // This is formulated to become an append (if there's no widget), an
     // insertion at index 0 (if there is a widget but no legend already), or
     // a no-op (if the legend is already in place).
     fieldset.insertBefore(legend, fieldset.firstChild);
   } else if (legend.parent != null) {
     // We remove the legend from the DOM because leaving it in with an empty
     // string renders as an ugly gap in the top border on some browsers.
     //fieldset.removeChild(legend);
     legend.remove();
   }
 }
}