form method
This attribute takes the value of the id attribute
of a <form> element you want the <fieldset> to be part
of, even if it is not inside the form.
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Implementation
FieldSet form(String id, {bool? removeIf}) {
if (removeIf != true && node.attrs != null) {
VirtualAttr attr = VirtualAttr('form', id);
node.attrs!.add(attr);
}
return this;
}