Fix bad handling of done events in _ForwardingStream.

Fix bad handling of done events in _ForwardingStream.

On a done event, the subscription is stale, but the forwarding
stream uses the presence of the subscription as a state-marker for
it being useful. Changed to null the subscription on a done event.

BUG=http://dartbug.com/8311

Committed: https://code.google.com/p/dart/source/detail?r=18212

[floitsch, 2013-02-05 15:09:51]

LGTM. but the whole setup is currently very difficult to follow. We should clean
this up and make things easier to reason about.

https://codereview.chromium.org/12213009/diff/3001/tests/lib/async/stream_tra...
File tests/lib/async/stream_transform_test.dart (right):

https://codereview.chromium.org/12213009/diff/3001/tests/lib/async/stream_tra...
tests/lib/async/stream_transform_test.dart:29: actual.onDone(() {
expectAsync0

https://codereview.chromium.org/12213009/diff/3001/tests/lib/async/stream_tra...
tests/lib/async/stream_transform_test.dart:48: }
more tests! :)
some other transformations would be nice.
map.map
...
simple stuff, but just to get a little bit more coverage.

[Lasse Reichstein Nielsen, 2013-02-07 10:16:58]

https://codereview.chromium.org/12213009/diff/3001/tests/lib/async/stream_tra...
File tests/lib/async/stream_transform_test.dart (right):

https://codereview.chromium.org/12213009/diff/3001/tests/lib/async/stream_tra...
tests/lib/async/stream_transform_test.dart:29: actual.onDone(() {
On 2013/02/05 15:09:51, floitsch wrote:
> expectAsync0

Done.

https://codereview.chromium.org/12213009/diff/3001/tests/lib/async/stream_tra...
tests/lib/async/stream_transform_test.dart:48: }
That won't give any more coverage for this bug. It's in their common superclass,
so it makes no difference which two you use.
I've added a few more to the existing case, to cover chains longer than two.