Renames the isolate's mutator_thread_ to scheduled_mutator_thread_

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 6bbc04d0778a13c02acf3e4972f712a040bea699..437d37878ce54204a9df24e64af73f520a93a805 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -218,9 +218,7 @@ class Isolate : public BaseIsolate {
   // Mutator thread is not scheduled if NULL or no heap is attached
   // to it. The latter only occurs when the mutator thread object
   // is unscheduled by the isolate (or never scheduled).
-  bool IsMutatorThreadScheduled() {
-    return mutator_thread_ != NULL && mutator_thread_->heap() != NULL;
-  }
+  bool IsMutatorThreadScheduled() { return scheduled_mutator_thread_ != NULL; }
 
   const char* name() const { return name_; }