union method
Set<String> union(Returns a new set which contains all the elements of this set and other.
That is, the returned set contains all the elements of this Set and all the elements of other.
Source
Set<String> union(Set<String> other) => readClasses().union(other);
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https://api.dartlang.org/stable/1.24.3/dart-svg/AttributeClassSet/union.html