Fix async iterator runtime type errors

Fix async iterator runtime type errors

Seeing errors on the bots triggered by a recent SDK change (FutureOrT typing?).  See travis for current breakage.

TBR=jmesserly@google.com

Committed: https://github.com/dart-lang/sdk/commit/c3505f7d7242f91717e8f0751a014007503a60e8

[vsm, 2017-03-23 00:15:14]

Description was changed from

==========
Fix async iterator runtime type errors

Seeing errors on the bots triggered by a recent SDK change.
==========

to

==========
Fix async iterator runtime type errors

Seeing errors on the bots triggered by a recent SDK change (FutureOrT typing?). 
See travis for current breakage.
==========

[vsm, 2017-03-23 00:15:14]

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

[Jennifer Messerly, 2017-03-23 00:31:20]

https://codereview.chromium.org/2769883002/diff/1/pkg/dev_compiler/tool/input...
File pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart
(right):

https://codereview.chromium.org/2769883002/diff/1/pkg/dev_compiler/tool/input...
pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart:52: future =
FutureT.value(future);
Chatted w/ Vijay offline ... I don't think this is right, because this is legal
in Dart:

Future<int> f() async {
  var a = await 'hello';
  var b = await 'world'
  return (a + b).length; // 10
}


... so we need to have the final Future be typed Future<T>, while allowing the
intermediate ones to be Future<top type> ... it may be that we can't use our
implementation like this anymore and we need to track the "final" one separately
with a Completer. IIRC that's how dart2js implements it.

[vsm, 2017-03-23 03:14:10]

Description was changed from

==========
Fix async iterator runtime type errors

Seeing errors on the bots triggered by a recent SDK change (FutureOrT typing?). 
See travis for current breakage.
==========

to

==========
Fix async iterator runtime type errors

Seeing errors on the bots triggered by a recent SDK change (FutureOrT typing?). 
See travis for current breakage.

TBR=jmesserly@google.com
==========

[vsm, 2017-03-23 03:14:34]

Fixed to type only the last future.

TBR'ing to get travis green.

[vsm, 2017-03-23 03:15:13]

Description was changed from

==========
Fix async iterator runtime type errors

Seeing errors on the bots triggered by a recent SDK change (FutureOrT typing?). 
See travis for current breakage.

TBR=jmesserly@google.com
==========

to

==========
Fix async iterator runtime type errors

Seeing errors on the bots triggered by a recent SDK change (FutureOrT typing?). 
See travis for current breakage.

TBR=jmesserly@google.com

Committed:
https://github.com/dart-lang/sdk/commit/c3505f7d7242f91717e8f0751a014007503a60e8
==========

[vsm, 2017-03-23 03:15:15]

Committed patchset #2 (id:10001) manually as
c3505f7d7242f91717e8f0751a014007503a60e8 (presubmit successful).

[Jennifer Messerly, 2017-03-23 19:10:15]

LGTM!