isStrictSubsetOf method
Returns true if every element of this is contained in other and at
least one element of other is not contained in this.
Example:
var set = {'a', 'b', 'c'};
set.isStrictSubsetOf({'a', 'b', 'c', 'd'}); // true
set.isStrictSubsetOf({'a', 'b', 'c'}); // false
set.isStrictSubsetOf({'a', 'b', 'f'}); // false
Implementation
bool isStrictSubsetOf(Set<Object> other) =>
this.length < other.length && other.containsAll(this);