Re-remove ad hoc Future.then inference.

Re-remove ad hoc Future.then inference.

Ad hoc inference for Future.then (and overrides) was kept in place for
an extra release cycle to allow code that implemented the Future
interface to work both before and after the transition to using
FutureOr for the callback return type.  This CL removes support for
this ad hoc inference.

BUG=
R=jmesserly@google.com

Committed: https://github.com/dart-lang/sdk/commit/006c22d0a271ae2ec38fe8124aeb8b071729f651

[Leaf, 2017-05-04 00:32:19]

leafp@google.com changed reviewers:
+ jmesserly@google.com

[Jennifer Messerly, 2017-05-04 00:36:37]

lgtm.then((_) => lgtm ? lgtm : new Future.value(lgtm))  =D

[Leaf, 2017-05-04 17:45:30]

Description was changed from

==========
Re-remove ad hoc Future.then inference.

Ad hoc inference for Future.then (and overrides) was kept in place for
an extra release cycle to allow code that implemented the Future
interface to work both before and after the transition to using
FutureOr for the callback return type.  This CL removes support for
this ad hoc inference.

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

to

==========
Re-remove ad hoc Future.then inference.

Ad hoc inference for Future.then (and overrides) was kept in place for
an extra release cycle to allow code that implemented the Future
interface to work both before and after the transition to using
FutureOr for the callback return type.  This CL removes support for
this ad hoc inference.

BUG=
R=jmesserly@google.com

Committed:
https://github.com/dart-lang/sdk/commit/006c22d0a271ae2ec38fe8124aeb8b071729f651
==========

[Leaf, 2017-05-04 17:45:31]

Committed patchset #2 (id:20001) manually as
006c22d0a271ae2ec38fe8124aeb8b071729f651 (presubmit successful).