Fix static warning in a test.

Fix static warning in a test.

Need to import dart:async to use Future class.

TBR=kasperl@google.com

Committed: https://code.google.com/p/dart/source/detail?r=29743

[fschneider, 2013-11-01 11:07:55]

Committed patchset #1 manually as r29743.

[Florian Schneider, 2013-11-01 11:09:23]

TBR

[kasperl, 2013-11-01 11:34:15]

LGTM.