Remove unneeded `strong_mode_down_cast_composite` ignores.

Remove unneeded `strong_mode_down_cast_composite` ignores.

As `strong_mode_down_cast_composite` is now a strong-mode hint these should no longer need to be explicitly ignored.

Follow-up from: https://github.com/flutter/flutter/pull/8853

BUG=
R=paulberry@google.com

Committed: https://github.com/dart-lang/sdk/commit/09886f491fa7ce21b7956b3bd8882983956ceeac

[pquitslund, 2017-03-17 17:22:02]

pquitslund@google.com changed reviewers:
+ ahe@google.com, paulberry@google.com

[Paul Berry, 2017-03-17 19:10:21]

lgtm

[pquitslund, 2017-03-17 19:31:50]

Description was changed from

==========
Remove unneeded `strong_mode_down_cast_composite` ignores.

As `strong_mode_down_cast_composite` is now a strong-mode hint these housld no
longer need to be explicitly ignored.

Follow-up from: https://github.com/flutter/flutter/pull/8853

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

to

==========
Remove unneeded `strong_mode_down_cast_composite` ignores.

As `strong_mode_down_cast_composite` is now a strong-mode hint these should no
longer need to be explicitly ignored.

Follow-up from: https://github.com/flutter/flutter/pull/8853

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

[pquitslund, 2017-03-17 19:33:05]

Description was changed from

==========
Remove unneeded `strong_mode_down_cast_composite` ignores.

As `strong_mode_down_cast_composite` is now a strong-mode hint these should no
longer need to be explicitly ignored.

Follow-up from: https://github.com/flutter/flutter/pull/8853

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

to

==========
Remove unneeded `strong_mode_down_cast_composite` ignores.

As `strong_mode_down_cast_composite` is now a strong-mode hint these should no
longer need to be explicitly ignored.

Follow-up from: https://github.com/flutter/flutter/pull/8853

BUG=
R=paulberry@google.com

Committed:
https://github.com/dart-lang/sdk/commit/09886f491fa7ce21b7956b3bd8882983956ceeac
==========

[pquitslund, 2017-03-17 19:33:09]

Committed patchset #1 (id:1) manually as
09886f491fa7ce21b7956b3bd8882983956ceeac (presubmit successful).