add nonempty check; don't trust Iterables

[jvasileff]

For most non-empty iterable-to-sequence operations where the iterable is not actually empty, there is a class cast exception. But this one trips up the Dart backend:

    object lyingIterable satisfies {Integer+} {
        iterator() => emptyIterator;
    }
    Integer x = [1, *lyingIterable][1];
    print(x); // <null>

It may be best to explicitly check all spread arguments involving non-empty Iterables.