Add IsMutatorThread to the Thread class and use it instead of MutatorThreadIsCurrentThread

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 86575f01ffb0c2a0b0df8e0976ad7be890524609..692351b884d5412f85b9ca289feace2976e90caa 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -151,8 +151,8 @@ class Isolate : public BaseIsolate {
   bool HasMutatorThread() {
     return mutator_thread_ != NULL;
   }
-  bool MutatorThreadIsCurrentThread() {
-    return mutator_thread_ == Thread::Current();
+  Thread* mutator_thread() const {
+    return mutator_thread_;
   }
 
   const char* name() const { return name_; }