clone method
Implementation
CSSStyleSheet clone() {
final clonedRules = cssRules.map(_cloneRuleForDiff).toList(growable: false);
CSSStyleSheet sheet = CSSStyleSheet(List.from(clonedRules), disabled: disabled, href: href);
for (final rule in sheet.cssRules) {
_assignParentStyleSheetRecursive(rule, sheet);
}
return sheet;
}