The signature of a generic comparison function.
A comparison function represents an ordering on a type of objects. A total ordering on a type means that for two values, either they are equal or one is greater than the other (and the latter must then be smaller than the former).
A Comparator function represents such a total ordering by returning
a is smaller than b,a is equal to b, anda is greater than b.
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https://api.dartlang.org/stable/2.0.0/dart-core/Comparator.html