Add framework for Dart_Terminate which will cleanup stuff on exit.

Fix for 12312 (Dartium crash when reloading after running tests).

Add framework for Dart_Cleanup which will cleanup stuff on exit.
Ideally the VM isolate should be shutdown in Dart_Terminate but we don't
seem to have a clean thread pool shutdown as a result of which there are
racing isolates.

R=iposva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=28248

[Ivan Posva, 2013-10-03 21:53:28]

LGTM

Maybe the less involved solution would have been to not unregister the code
observers.

-Ivan

[siva, 2013-10-03 21:57:44]

On 2013/10/03 21:53:28, Ivan Posva wrote:
> LGTM
> 
> Maybe the less involved solution would have been to not unregister the code
> observers.
> 
> -Ivan

Some of the CodeObservers (like perf) do the file write in the destructor which
gets
executed when they are unregistered.

[siva, 2013-10-03 22:17:54]

Committed patchset #2 manually as r28248 (presubmit successful).