Fix for returned Futures in unittest

Fix for returned Futures in unittest

BUG=https://code.google.com/p/dart/issues/detail?id=9344

Committed: https://code.google.com/p/dart/source/detail?r=20337

[kevmoo-old, 2013-03-21 17:23:03]

See the example bug for more simple repo.

Turns out TestCase was not returning the chained future call which handled
errors.

For some crazy reason, the crash doesn't surface unless there is a 
Future-returning test (with any result) that follows a unit test that returning
a Future as exception.

This fix allows a major cleanup in the test cases. The _defer method no longer
has to wrap all functions with guardAsync()

I also took the time to cleanup unittest_test to use convenience methods from
Future instead of Completer

[gram, 2013-03-21 17:55:24]

lgtm

lgtm with one comment.

https://codereview.chromium.org/12836007/diff/5001/pkg/unittest/test/unittest...
File pkg/unittest/test/unittest_test.dart (right):

https://codereview.chromium.org/12836007/diff/5001/pkg/unittest/test/unittest...
pkg/unittest/test/unittest_test.dart:153: });
Can you explain why you added the defer here? And if it was for a good reason,
can you add the explanation as a comment, as it isn't clear to me.

[kevmoo-old, 2013-03-21 18:19:57]

Comment on Gram's nit. Swapped Siggi for Bob as reviewers.

https://codereview.chromium.org/12836007/diff/5001/pkg/unittest/test/unittest...
File pkg/unittest/test/unittest_test.dart (right):

https://codereview.chromium.org/12836007/diff/5001/pkg/unittest/test/unittest...
pkg/unittest/test/unittest_test.dart:153: });
It ends up being a nice little verification of expectAsync0.

Might be slightly redundant to other tests, but it's not hurting anything. The
simplified version tests less, so I vote we keep it.

[kevmoo-old, 2013-03-21 18:22:28]

Committed patchset #3 manually as r20337 (presubmit successful).