Fix race condition with HasMutatorThread that was being used without a lock.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index b49d59b130bf265f9f316dee4d2d03b39ab94e08..97610713f0d037987f19e5a04d65992df197fc57 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -177,15 +177,7 @@ class Isolate : public BaseIsolate {
     message_notify_callback_ = value;
   }
 
-  // Limited public access to BaseIsolate::mutator_thread_ for code that
-  // must treat the mutator as the default or a special case. Prefer code
-  // that works uniformly across all threads.
-  bool HasMutatorThread() {
-    return mutator_thread_ != NULL;
-  }
-  Thread* mutator_thread() const {
-    return mutator_thread_;

[Florian Schneider, 2016/07/28 18:13:36]

Can you remove mutator_thread_ in BaseIsolate? It seems unused now.

[siva, 2016/07/28 20:15:10]

The field is still used in EnterIsolate to ensure that we don't have multiple
mutator threads entering an Isolate (see Isolate::ScheduleThread). That check is
under a lock.

-  }
+  Thread* mutator_thread() const;
 
   const char* name() const { return name_; }
   const char* debugger_name() const { return debugger_name_; }
@@ -466,8 +458,7 @@ class Isolate : public BaseIsolate {
 
   // Mutator thread is used to aggregate compiler stats.
   CompilerStats* aggregate_compiler_stats() {
-    ASSERT(HasMutatorThread());
-    return mutator_thread_->compiler_stats();
+    return mutator_thread()->compiler_stats();
   }
 
   VMTagCounters* vm_tag_counters() {
@@ -693,8 +684,8 @@ class Isolate : public BaseIsolate {
   // DEPRECATED: Use Thread's methods instead. During migration, these default
   // to using the mutator thread (which must also be the current thread).
   Zone* current_zone() const {
-    ASSERT(Thread::Current() == mutator_thread_);
-    return mutator_thread_->zone();
+    ASSERT(Thread::Current() == mutator_thread());
+    return mutator_thread()->zone();
   }
 
   // Accessed from generated code: