Don't cleanup thread list mutex until VM shutdown is clean

Don't cleanup thread list mutex until VM shutdown is clean

- We had a race where the thread pool would be starting up a thread while Dart::Cleanup was running on the main thread.
- The mutex used to guard the thread list was being deleted and set to NULL.
- Thread would try and add itself to the list and die when attempting to lock the mutex.

This CL does two things:

- Moves Thread list mutex cleanup to only run under FLAG_shutdown.
- Fixes a forgotten NULL initilization of thread list mutex.

R=whesse@google.com

Committed: https://github.com/dart-lang/sdk/commit/7803350a23beb025739c00ba19859a826d556926

[Cutch, 2015-10-13 21:35:51]

johnmccutchan@google.com changed reviewers:
+ asiva@google.com, whesse@google.com

[Bill Hesse, 2015-10-13 21:41:10]

lgtm

[Cutch, 2015-10-13 21:44:15]

Committed patchset #1 (id:1) manually as
7803350a23beb025739c00ba19859a826d556926 (presubmit successful).

[siva, 2015-10-13 21:44:20]

lgtm