Restore weak mode errors on empty returns in Null functions

Restore weak mode errors on empty returns in Null functions.

Fixes https://github.com/dart-lang/sdk/issues/28668 .

BUG=
R=brianwilkerson@google.com

Committed: https://github.com/dart-lang/sdk/commit/e40f878abc807ac2a0bb81bc268aa78a1da642e2

[Leaf, 2017-03-17 20:59:38]

Description was changed from

==========
Restore weak mode errors on empty returns in Null functions

BUG=
==========

to

==========
Restore weak mode errors on empty returns in Null functions.

Fixes https://github.com/dart-lang/sdk/issues/28668 .

BUG=
==========

[Leaf, 2017-03-17 20:59:38]

leafp@google.com changed reviewers:
+ brianwilkerson@google.com, lrn@google.com

[Brian Wilkerson, 2017-03-17 21:23:05]

lgtm

[Leaf, 2017-03-17 21:31:02]

Description was changed from

==========
Restore weak mode errors on empty returns in Null functions.

Fixes https://github.com/dart-lang/sdk/issues/28668 .

BUG=
==========

to

==========
Restore weak mode errors on empty returns in Null functions.

Fixes https://github.com/dart-lang/sdk/issues/28668 .

BUG=
R=brianwilkerson@google.com

Committed:
https://github.com/dart-lang/sdk/commit/e40f878abc807ac2a0bb81bc268aa78a1da642e2
==========

[Leaf, 2017-03-17 21:31:08]

Committed patchset #1 (id:1) manually as
e40f878abc807ac2a0bb81bc268aa78a1da642e2 (presubmit successful).