Avoid warning when using `return;` from sync* and async* functions.

Avoid warning when using `return;` from sync* and async* functions.

Fixes https://github.com/dart-lang/sdk/issues/24406

R=johnniwinther@google.com

Committed: https://github.com/dart-lang/sdk/commit/75f80a204b98ebd65a5da1f2c1db6c6388d67887
Reverted: https://github.com/dart-lang/sdk/commit/8163c4a14199314b6fad626de61d031a35b4d2c1

Committed: https://github.com/dart-lang/sdk/commit/1c3dd67408df7273d7b71915569ab8a7ef2a67aa

[sigurdm, 2015-09-23 08:04:38]

sigurdm@google.com changed reviewers:
+ johnniwinther@google.com

[Johnni Winther, 2015-09-23 08:08:23]

LGTM but also add a test to the type_checker_test.dart unittest.

[sigurdm, 2015-09-23 08:33:30]

Patchset #3 (id:40001) has been deleted

[sigurdm, 2015-09-23 08:34:23]

Patchset #3 (id:60001) has been deleted

[sigurdm, 2015-09-23 08:34:57]

ptal

[Johnni Winther, 2015-09-23 09:00:43]

lgtm

[sigurdm, 2015-09-23 09:07:43]

Committed patchset #3 (id:80001) manually as
75f80a204b98ebd65a5da1f2c1db6c6388d67887 (presubmit successful).

[sigurdm, 2015-09-23 10:43:30]

ptal

[Johnni Winther, 2015-09-23 11:09:46]

lgtm

[sigurdm, 2015-09-23 11:51:08]

Committed patchset #4 (id:100001) manually as
1c3dd67408df7273d7b71915569ab8a7ef2a67aa (presubmit successful).