Make the lock/unlock and Enter/Exit methods in Mutex and Monitor private so taht we always use the …

runtime/vm/service.cc

Index: runtime/vm/service.cc
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index b7ac72158c8937a420117ecc08ddea19b9d6a375..70e677a0df4befa7158c1e7f7e7de06b92ab103b 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -1291,8 +1291,7 @@ static bool GetStack(Thread* thread, JSONStream* js) {
   }
 
   {
-    MessageHandler::AcquiredQueues aq;
-    isolate->message_handler()->AcquireQueues(&aq);
+    MessageHandler::AcquiredQueues aq(isolate->message_handler());
     jsobj.AddProperty("messages", aq.queue());
   }
 
@@ -1679,8 +1678,7 @@ static RawObject* LookupHeapObjectMessage(Thread* thread,
   if (!GetUnsignedIntegerId(parts[1], &message_id, 16)) {
     return Object::sentinel().raw();
   }
-  MessageHandler::AcquiredQueues aq;
-  thread->isolate()->message_handler()->AcquireQueues(&aq);
+  MessageHandler::AcquiredQueues aq(thread->isolate()->message_handler());
   Message* message = aq.queue()->FindMessageById(message_id);
   if (message == NULL) {
     // The user may try to load an expired message.