Adjust acceptable wake-up delay in Monitor test.

Adjust acceptable wake-up delay in Monitor test.

The monitor wake-up delay is occasionally a bit more than 120 ms
on Windows.

Example flake:
http://build.chromium.org/p/client.dart/builders/vm-win32-release/builds/2353/steps/tests/logs/stdio

R=sgjesse@google.com,iposva@google.com
BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=6836

[Søren Gjesse, 2012-04-23 11:29:14]

lgtm