Add map transition for observed objects

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 3a60a322a139ad3c495423cb05ca4581bd3662f3..2ea4aab572a02bf3c493276bf25c97d809105f0f 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -6671,6 +6671,37 @@ MaybeObject* Map::CopyAsElementsKind(ElementsKind kind, TransitionFlag flag) {
 }
 
 
+MaybeObject* Map::CopyForObserved() {
+  ASSERT(!is_observed());
+
+  Map* new_map;
+  MaybeObject* maybe_new_map;
+  if (owns_descriptors()) {
+    maybe_new_map = CopyDropDescriptors();
+  } else {
+    maybe_new_map = Copy();
+  }
+  if (!maybe_new_map->To(&new_map)) return maybe_new_map;
+
+  TransitionArray* transitions;
+  MaybeObject* maybe_transitions = AddTransition(GetHeap()->observed_symbol(),
+                                                 new_map,
+                                                 FULL_TRANSITION);
+  if (!maybe_transitions->To(&transitions)) return maybe_transitions;
+  set_transitions(transitions);
+
+  new_map->set_is_observed(true);
+
+  if (owns_descriptors()) {
+    new_map->InitializeDescriptors(instance_descriptors());
+    set_owns_descriptors(false);
+  }
+
+  new_map->SetBackPointer(this);
+  return new_map;
+}
+
+
 MaybeObject* Map::CopyWithPreallocatedFieldDescriptors() {
   if (pre_allocated_property_fields() == 0) return CopyDropDescriptors();