Make the spec specific about the return type of sync*/async/async* function.

Make the spec specific about the return type of sync*/async/async* function.

Now requires the type to be a supertype of `Iterable<T>`/`Future<T>`/`Stream<T>`
for some `T`. This allows, say, all `Future<T>` return types for `async` but
disallows using a different class implementing `Future`:
```
MyFuture foo() async {}
```
gives a static warning (error in Dart 2) because the return type is statically
known to be unsatisfied.

Fixes issue #27470
BUG= http://dartbugcom/27470
R=eernst@google.com, floitsch@google.com

Committed: https://github.com/dart-lang/sdk/commit/16d806b0aa4b1100e1dffe6940e80014070b1bba

[Lasse Reichstein Nielsen, 2016-10-03 12:00:10]

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

[Lasse Reichstein Nielsen, 2016-10-03 12:00:11]

WDYT?

[Lasse Reichstein Nielsen, 2016-10-03 12:00:28]

WDYT?

[eernst, 2016-10-03 12:49:08]

LGTM

https://codereview.chromium.org/2392513002/diff/1/docs/language/dartLangSpec.tex
File docs/language/dartLangSpec.tex (right):

https://codereview.chromium.org/2392513002/diff/1/docs/language/dartLangSpec....
docs/language/dartLangSpec.tex:597: It is a static warning if the declared
return type of a function marked \ASYNC* is not a supertype of
\code{Stream$<$$T$$>$} for some type $T$.
OK: Discussed IRL, and I agree that this does not eliminate any useful programs.
Consider using `$<\mbox{$T$}>$` to control the spacing.

[floitsch, 2016-10-03 14:11:00]

LGTM.

[Lasse Reichstein Nielsen, 2016-10-05 08:22:57]

https://codereview.chromium.org/2392513002/diff/1/docs/language/dartLangSpec.tex
File docs/language/dartLangSpec.tex (right):

https://codereview.chromium.org/2392513002/diff/1/docs/language/dartLangSpec....
docs/language/dartLangSpec.tex:597: It is a static warning if the declared
return type of a function marked \ASYNC* is not a supertype of
\code{Stream$<$$T$$>$} for some type $T$.
Changed to using mbox.

[Lasse Reichstein Nielsen, 2016-10-05 08:25:46]

Description was changed from

==========
Make the spec specific about the return type of sync*/async/async* function.

Now requires the type to be a supertype of `Iterable<T>`/`Future<T>`/`Stream<T>`
for some `T`. This allows, say, all `Future<T>` return types for `async` but
disallows using a different class implementing `Future`:
```
MyFuture foo() async {}
```
gives a static warning (error in Dart 2) because the return type is statically
known to be unsatisfied.

Fixes issue #27470
BUG= http://dartbugcom/27470
==========

to

==========
Make the spec specific about the return type of sync*/async/async* function.

Now requires the type to be a supertype of `Iterable<T>`/`Future<T>`/`Stream<T>`
for some `T`. This allows, say, all `Future<T>` return types for `async` but
disallows using a different class implementing `Future`:
```
MyFuture foo() async {}
```
gives a static warning (error in Dart 2) because the return type is statically
known to be unsatisfied.

Fixes issue #27470
BUG= http://dartbugcom/27470
R=eernst@google.com, floitsch@google.com

Committed:
https://github.com/dart-lang/sdk/commit/16d806b0aa4b1100e1dffe6940e80014070b1bba
==========

[Lasse Reichstein Nielsen, 2016-10-05 08:25:48]

Committed patchset #2 (id:20001) manually as
16d806b0aa4b1100e1dffe6940e80014070b1bba (presubmit successful).