Fix type of EninePlugin List<ResultDescriptor> getters.

Fix type of EninePlugin List<ResultDescriptor> getters.

These getters were improperly typed as List<TaskDescriptor>.  Due to the
fact that the underlying lists they were returning were List<dynamic>,
and the fact that new warnings for for-in loops haven't been implemented
yet (see commit 89607f06e9ccf9274f8ada2ecd4d004dbee9fb53), no warnings
or runtime errors alerted us to the incorrect type.

R=brianwilkerson@google.com

Committed: https://github.com/dart-lang/sdk/commit/ec144a055abb377e8168e6bd53370b137508d727

[Paul Berry, 2015-10-13 16:18:23]

paulberry@google.com changed reviewers:
+ brianwilkerson@google.com

[Brian Wilkerson, 2015-10-13 17:29:24]

LGTM

[Paul Berry, 2015-10-13 17:45:33]

Committed patchset #1 (id:1) manually as
ec144a055abb377e8168e6bd53370b137508d727 (presubmit successful).