Fix File stream on cancel.

Fix File stream on cancel.

Before we could end up having both an issued read and a close request.
Given the new nature of our io-service, this could lead to a race
condition between the two.

BUG=https://code.google.com/p/dart/issues/detail?id=12982
R=sgjesse@google.com

Committed: https://code.google.com/p/dart/source/detail?r=27955

[Søren Gjesse, 2013-09-26 14:28:38]

LGTM

Don't we have the same kind of issues with RandomAccessFile, where a close can
take over other dispatched operations?

[Anders Johnsen, 2013-09-26 14:33:28]

We do. Should we add a boolean to mark when it's being used, so we can throw an
exception?

[Anders Johnsen, 2013-09-26 14:34:32]

Committed patchset #4 manually as r27955 (presubmit successful).