Move no_callback_scope_depth_ and cha_ from isolate -> thread

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 269253aef321b781c93fbaef4c4483d02f72299e..6b4264b12231f0fa4e575757e107c4e653bbea6e 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -1826,7 +1826,9 @@ RawObject* Object::Allocate(intptr_t cls_id,
   ASSERT(Utils::IsAligned(size, kObjectAlignment));
   Thread* thread = Thread::Current();
   Isolate* isolate = thread->isolate();
-  ASSERT(isolate->no_callback_scope_depth() == 0);
+  // New space allocation allowed only in mutator thread (Dart thread);
+  ASSERT(isolate->MutatorThreadIsCurrentThread() || (space != Heap::kNew));
+  ASSERT(thread->no_callback_scope_depth() == 0);
   Heap* heap = isolate->heap();
 
   uword address = heap->Allocate(size, space);