Fix bug 846:

Fix bug 846:
- Only execute timers which were pending prior to the wakeup to
  avoid busy looping.
- Prevent double notification for timers.
Committed: https://code.google.com/p/dart/source/detail?r=2500

[siva, 2011-12-16 01:42:02]

http://codereview.chromium.org/8973005/diff/1/bin/timer_impl.dart
File bin/timer_impl.dart (right):

http://codereview.chromium.org/8973005/diff/1/bin/timer_impl.dart#newcode30
bin/timer_impl.dart:30: timer._notifyEventHandler();
We need to call notifyEventHandler only if addTimerToList added this new timer
object as the first element of the timer list. This would further cut down calls
to notifyEventHandler.

[siva, 2011-12-16 01:43:16]

lgtm