Keep StoreBuffer::global_mutex_ and global_empty_ alive.
Keep StoreBuffer::global_mutex_ and global_empty_ alive.

At Dart::CleanUp time, there may still be threads around that access their store buffer, so we need to keep these around until @zanderso lands the clean shutdown CL.

We should stop leaking them once we cleanly stop all threads at VM shutdown (see issue 23844).

BUG=https://github.com/dart-lang/sdk/issues/24169
R=iposva@google.com

Review URL: https://codereview.chromium.org//1305123003 .

dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk