Make Completer.complete handle a Future with a misbehaving "then" call.

Make Completer.complete handle a Future with a misbehaving "then" call.

Instead of throwing and getting into an invalid state (isChained but won't ever
complete), the synchronous error from trying to call "then" is used to complete
the future.

This should never happen in practice - it requires a "then" implementation that
is missing or throwing even when given valid arguments.

R=sgjesse@google.com

Committed: https://code.google.com/p/dart/source/detail?r=44629

[Lasse Reichstein Nielsen, 2015-03-23 09:02:34]

lrn@google.com changed reviewers:
+ sgjesse@google.com, sigurdm@google.com

[Søren Gjesse, 2015-03-23 09:07:36]

lgtm

https://codereview.chromium.org/1025133004/diff/1/sdk/lib/async/future_impl.dart
File sdk/lib/async/future_impl.dart (right):

https://codereview.chromium.org/1025133004/diff/1/sdk/lib/async/future_impl.d...
sdk/lib/async/future_impl.dart:321: // The `then` call threw synchronously. This
should never happen!
Maybe explain "This should never happen!" - I assume this is the case for the
_Future implementation.

[Lasse Reichstein Nielsen, 2015-03-23 09:09:39]

Committed patchset #2 (id:20001) manually as 44629 (presubmit successful).

[Lasse Reichstein Nielsen, 2015-03-23 09:43:54]

https://codereview.chromium.org/1025133004/diff/1/sdk/lib/async/future_impl.dart
File sdk/lib/async/future_impl.dart (right):

https://codereview.chromium.org/1025133004/diff/1/sdk/lib/async/future_impl.d...
sdk/lib/async/future_impl.dart:321: // The `then` call threw synchronously. This
should never happen!
The _Future implementation never reaches here. To even reach this function, a
different implementation of Future is required, and to get to the catch, it must
throw when calling "then" with perfectly good arguments - so it's a really bad
implementation - which obviously *shouldn't* happen.