Make stream_timeout_test less likely to flake.

Make stream_timeout_test less likely to flake.

R=lrn@google.com

Committed: https://code.google.com/p/dart/source/detail?r=33214

[floitsch, 2014-03-03 13:06:39]

I gave the test more time (although 500ms already seemed plenty). Since we don't
really know why the timeout triggered, we might encounter it again. But then we
really know that IE9 is just acting up, and should mark the test as flaky.

Flaked stdio:
http://build.chromium.org/p/client.dart/builders/dart2js-full-windows-7-8-be/...

[Lasse Reichstein Nielsen, 2014-03-03 13:16:15]

lgtm

[floitsch, 2014-03-03 13:18:36]

Committed patchset #1 manually as r33214 (presubmit successful).

[ricow1, 2014-03-03 13:20:00]

thanks