Mark timer_test as failing on Dartium.

Mark timer_test as failing on Dartium.

After removing the 1 ms fiddling factor from the test, it still
fails on Dartium. It does not fail on the VM any more, since the
timer was fixed, so I'd prefer to not change the test back,
since t really does detect an error.

The problem is that the timer fires too soon.
The difference between a DateTime.now() created before
the timer was started, and one created after the time completed,
gives a time that is less than the timeout.

It may be due to either DateTime.now not being good enough, or the
timer firing too early, but it's likely the latter.

R=sgjesse@google.com

Committed: https://code.google.com/p/dart/source/detail?r=31061

[Søren Gjesse, 2013-12-11 10:14:17]

lgtm

[Lasse Reichstein Nielsen, 2013-12-11 10:18:37]

Committed patchset #1 manually as r31061 (presubmit successful).