isValidKey method
- Object? o
override
    Test whether an object is a valid argument to equals and hash.
Some implementations may be restricted to only work on specific types of objects.
Implementation
@override
bool isValidKey(Object? o) {
  if (o is E) {
    final value = _comparisonKey(o);
    return _inner.isValidKey(value);
  }
  return false;
}