Add 'timeout' for io_test.
Add 'timeout' for io_test.

This reintroduces the change from https://codereview.chromium.org/11648033/
which was reverted in https://codereview.chromium.org/11570065/.

Review URL: https://codereview.chromium.org//11660003

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@16425 260f80e4-7a28-3924-810f-c04153c831b5

cjkao/dart-pub,dart-lang/pub,computmaxer/pub,dart-lang/pub,maxwellpeterson-wf/pub