Create a hash set using the provided equals as equality.
The provided equals must define a stable equivalence relation, and
hashCode must be consistent with equals. If the equals or hashCode
methods won't work on all objects, but only on some instances of E, the
isValidKey predicate can be used to restrict the keys that the functions
are applied to.
Any key for which isValidKey returns false is automatically assumed
to not be in the set when asking contains.
If equals or hashCode are omitted, the set uses
the elements' intrinsic Object.operator== and Object.hashCode.
If you supply one of equals and hashCode,
you should generally also to supply the other.
If the supplied equals or hashCode functions won't work on all E
objects, and the map will be used in a setting where a non-E object
is passed to, e.g., contains, then the isValidKey function should
also be supplied.
If isValidKey is omitted, it defaults to testing if the object is an
E instance. That means that:
new HashSet<int>(equals: (int e1, int e2) => (e1 - e2) % 5 == 0,
hashCode: (int e) => e % 5)
does not need an isValidKey argument, because it defaults to only
accepting int values which are accepted by both equals and hashCode.
If neither equals, hashCode, nor isValidKey is provided,
the default isValidKey instead accepts all values.
The default equality and hashcode operations are assumed to work on all
objects.
Likewise, if equals is identical, hashCode is identityHashCode
and isValidKey is omitted, the resulting set is identity based,
and the isValidKey defaults to accepting all keys.
Such a map can be created directly using HashSet.identity.
Source
external factory HashSet({bool equals(E e1, E e2),
int hashCode(E e),
bool isValidKey(potentialKey)});