Make synchronous broadcast StreamController throw if adding event while adding event.

Make synchronous broadcast StreamController throw if adding event while adding event.

It already throws if there is more than one listener, but this change
also makes it throw if there is only one listener, for consistency.

Also add SynchronousStreamController abstract class.
It only contains documentation for synchronous stream controllers.

BUG= http://dartbug.com/22240
R=floitsch@google.com

Committed: https://code.google.com/p/dart/source/detail?r=43946

[Lasse Reichstein Nielsen, 2015-02-03 13:38:27]

lrn@google.com changed reviewers:
+ floitsch@google.com

[Lasse Reichstein Nielsen, 2015-02-03 13:38:27]

Maybe a little long-winded.

[floitsch, 2015-02-11 10:00:42]

LGTM.

https://codereview.chromium.org/882713009/diff/1/pkg/async/pubspec.yaml
File pkg/async/pubspec.yaml (right):

https://codereview.chromium.org/882713009/diff/1/pkg/async/pubspec.yaml#newcode2
pkg/async/pubspec.yaml:2: version: 1.0.1-dev
not sure if you don't want to go higher.

[Lasse Reichstein Nielsen, 2015-02-23 10:22:11]

https://codereview.chromium.org/882713009/diff/1/pkg/async/pubspec.yaml
File pkg/async/pubspec.yaml (right):

https://codereview.chromium.org/882713009/diff/1/pkg/async/pubspec.yaml#newcode2
pkg/async/pubspec.yaml:2: version: 1.0.1-dev
I'll go to 1.1. It's detectable - but only to programs that rely on behavior
that is counter to the documentation.

[Lasse Reichstein Nielsen, 2015-02-23 10:33:22]

New patchsets have been uploaded after l-g-t-m from floitsch@google.com

[Lasse Reichstein Nielsen, 2015-02-23 10:37:09]

Committed patchset #2 (id:20001) manually as 43946 (presubmit successful).

[Lasse Reichstein Nielsen, 2015-02-23 10:38:49]

https://codereview.chromium.org/882713009/diff/1/pkg/async/pubspec.yaml
File pkg/async/pubspec.yaml (right):

https://codereview.chromium.org/882713009/diff/1/pkg/async/pubspec.yaml#newcode2
pkg/async/pubspec.yaml:2: version: 1.0.1-dev
Hmm, that was unnecessary (but now it's done). The only change to the package
was in a test, which is not really a user visible change.