Ensure helper threads get empty initial store buffer block.

runtime/vm/thread.cc

Index: runtime/vm/thread.cc
diff --git a/runtime/vm/thread.cc b/runtime/vm/thread.cc
index 654d02ff9ebef8166d5929d4c019247c805334f5..fdd3272e340ecf1029b9a87c5c067bbcd3b80440 100644
--- a/runtime/vm/thread.cc
+++ b/runtime/vm/thread.cc
@@ -173,7 +173,9 @@ void Thread::EnterIsolateAsHelper(Isolate* isolate) {
   ASSERT(thread->isolate() == NULL);
   thread->isolate_ = isolate;
   ASSERT(thread->store_buffer_block_ == NULL);
-  thread->StoreBufferAcquire();
+  // TODO(koda): Use StoreBufferAcquire once we properly flush before Scavenge.
+  thread->store_buffer_block_ =
+      thread->isolate()->store_buffer()->PopEmptyBlock();
   ASSERT(isolate->heap() != NULL);
   thread->heap_ = isolate->heap();
   ASSERT(thread->thread_state() == NULL);