问如何在使用Stream.periodic时取消流？

EN

import 'dart:async';

void main() {
  int count = 0;
  final Stream newsStream = new Stream.periodic(Duration(seconds: 2), (_) {
    return _;
  });

  StreamSubscription mySubscribedStream = newsStream.map((e) {
    count = e;
    print(count);
    return 'stuff $e';
  }).listen((e) {
    print(e);
  });

  // count = 0 here because count is scoped inside mySubscribedStream
  // How do I extract out 'count', so I can cancel the stream?
  if (count > 5) {
    mySubscribedStream.cancel();
    mySubscribedStream = null;
  }
}