Fix reuse of free blocks and delete global cache on VM shutdown.

runtime/vm/store_buffer.cc

Index: runtime/vm/store_buffer.cc
diff --git a/runtime/vm/store_buffer.cc b/runtime/vm/store_buffer.cc
index 7fe58c74290511b3938a448735e28b89494a8940..ca1ff21bbf192269f5d4e80e81bc0206cacfa253 100644
--- a/runtime/vm/store_buffer.cc
+++ b/runtime/vm/store_buffer.cc
@@ -26,6 +26,12 @@ void StoreBuffer::InitOnce() {
 }
 
 
+void StoreBuffer::ShutDown() {
+  delete global_empty_;
+  delete global_mutex_;
+}
+
+
 StoreBuffer::StoreBuffer() : mutex_(new Mutex()) {
 }
 
@@ -105,7 +111,7 @@ StoreBufferBlock* StoreBuffer::PopEmptyBlock() {
   {
     MutexLocker ml(global_mutex_);
     if (!global_empty_->IsEmpty()) {
-      global_empty_->Pop();
+      return global_empty_->Pop();
     }
   }
   return new StoreBufferBlock();