main.dart

Iterable<BigInt> getFibIterator() sync* {
    var initial = true;
    BigInt prev = null;
    BigInt beforePrev = BigInt.from(1);
    while(true){
        if(initial && prev == null){
            initial = false;
            prev = BigInt.from(0);
            yield BigInt.from(0);
        } else {
            final temp = prev + beforePrev;
            prev = beforePrev;
            beforePrev = temp;
            yield prev;
        }
    }
}

class FibIterable extends  Iterable  {
    final _iterator = FibonacciIterator();

    @override
    Iterator get iterator => _iterator;
}


class FibonacciIterator implements Iterator<BigInt> {
    bool _initial = true;
    BigInt _prev = null;
    BigInt _beforePrev = BigInt.from(1);


    bool moveNext() {
        if (_initial && _prev == null) {
            _initial = false;
            _prev = BigInt.from(0);
        } else  {
            final BigInt _temp = _prev + _beforePrev;
            _prev = _beforePrev;
            _beforePrev = _temp;
        }
        return true;
    }

    BigInt get current => _prev;
}