Add VMService GC events

Add a new event family for GC-related events, and generate an event after each garbage collection with basic stats.

One reason for this is to provide a structured alternative to the ad hoc logging format of --verbose_gc, and to move some functionality currently provided by standalone one-off scripts into the observatory.

Add first unit test for events.

R=johnmccutchan@google.com

Committed: https://code.google.com/p/dart/source/detail?r=39014

[koda, 2014-08-05 05:00:27]

This CL doesn't include any Observatory UI for these events (while developing
I've relied on an annoying pop-up and dumping events to the log). I wanted to
get some early feedback on whether this is the proper way to add a new event
type.

I do have some UI ideas (also for generating particular graphs from the
historical GC stats, something currently done by one-off scripts) that I'd like
to discuss when we are both back in the office.

[koda, 2014-08-06 23:50:02]

Added unit test (first unit test for async events).

[Cutch, 2014-08-07 14:48:08]

lgtm

[koda, 2014-08-07 23:52:25]

Committed patchset #6 manually as r39014 (presubmit successful).