Reuse empty StoreBufferBlocks.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 2432df990bfa34e8945bc702086c827c4bef13c6..3810ee51529153e89971d6fd76c73babc26fc0d0 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -31,6 +31,7 @@
 #include "vm/service_isolate.h"
 #include "vm/simulator.h"
 #include "vm/stack_frame.h"
+#include "vm/store_buffer.h"
 #include "vm/stub_code.h"
 #include "vm/symbols.h"
 #include "vm/tags.h"
@@ -611,7 +612,7 @@ void BaseIsolate::AssertCurrent(BaseIsolate* isolate) {
 Isolate::Isolate(const Dart_IsolateFlags& api_flags)
   :   mutator_thread_(NULL),
       vm_tag_(0),
-      store_buffer_(),
+      store_buffer_(new StoreBuffer()),
       message_notify_callback_(NULL),
       name_(NULL),
       debugger_name_(NULL),
@@ -686,6 +687,7 @@ Isolate::Isolate(const Dart_IsolateFlags& api_flags)
 Isolate::~Isolate() {
   free(name_);
   free(debugger_name_);
+  delete store_buffer_;
   delete heap_;
   delete object_store_;
   delete api_state_;