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

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 2d8bb4bfb638e2538f48d17e92a99fb18f81e5b6..7abc24875e8326b68c05ac17eb201afa40e21d7f 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -2554,6 +2554,10 @@ Thread* Isolate::ScheduleThread(bool is_mutator, bool bypass_safepoint) {
     os_thread->set_thread(thread);
     if (is_mutator) {
       mutator_thread_ = thread;
+      if (this != Dart::vm_isolate()) {
+        mutator_thread_->set_top(heap()->new_space()->top());
+        mutator_thread_->set_end(heap()->new_space()->end());
+      }
     }
     Thread::SetCurrent(thread);
     os_thread->EnableThreadInterrupts();
@@ -2592,6 +2596,12 @@ void Isolate::UnscheduleThread(Thread* thread,
   os_thread->set_thread(NULL);
   OSThread::SetCurrent(os_thread);
   if (is_mutator) {
+    if (this != Dart::vm_isolate()) {
+      heap()->new_space()->set_top(mutator_thread_->top_);
+      heap()->new_space()->set_end(mutator_thread_->end_);
+    }
+    mutator_thread_->top_ = 0;
+    mutator_thread_->end_ = 0;
     mutator_thread_ = NULL;
   }
   thread->isolate_ = NULL;