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 f700bbc2ad2e2c944c3e506be5e297e54c91e171..37793daf0666fc61e9db050fc38c047046b05a68 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -220,6 +220,13 @@ 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);