single property
override
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
E get single {
if (isEmpty) {
throw StateError('No such element');
}
if (_length > 1) {
throw StateError('Too many elements');
}
return _first!;
}