Helper method used to modify the set of css classes on this element.
f - callback with: s - a Set of all the css class name currently on this element.
After f returns, the modified set is written to the className property of this element.
Source
modify(f(Set<String> s)) {
Set<String> s = readClasses();
var ret = f(s);
writeClasses(s);
return ret;
}