Moves the top_ and end_ words of the Scavenger into mutator thread.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 8dc5c5c3867952eff1ab857f4b344cdd52af8339..e5b1849d6999df4c3956dc5d275fd7fd6ee3ba6a 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -215,6 +215,13 @@ class Isolate : public BaseIsolate {
 
   Thread* mutator_thread() const;
 
+  // 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;
+  }
+
   const char* name() const { return name_; }
   const char* debugger_name() const { return debugger_name_; }
   void set_debugger_name(const char* name);