Bump wait time for watcher test.

This is to see if it makes the test less flaky on the bots,
in particular the ARM ones.

BUG=https://code.google.com/p/dart/issues/detail?id=13705
R=nweiz@google.com

Review URL: https://codereview.chromium.org//26398002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@28341 260f80e4-7a28-3924-810f-c04153c831b5