single property
Checks that this iterable has only one element, and returns that element.
Throws a StateError if this
is empty or has more than one element.
This operation will not iterate past the second element.
Implementation
@override
E get single {
try {
return _elements.single;
} on StateError catch (e) {
throw NoGuaranteeException('Queue is empty', cause: e);
}
}