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.
Source
Node get single {
int len = this.length;
if (len == 1) {
return _nativeIndexedGetter(0);
}
if (len == 0) throw new StateError("No elements");
throw new StateError("More than one element");
}