Promoting elements transitions to their own field.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index d18a158c44d0442eb42ec67c5bea514031c15b48..4c57c4c489e5ff7cb31f8dd46e44b6b76e931f43 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -1236,7 +1236,8 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_DisableAccessChecks) {
   if (needs_access_checks) {
     // Copy map so it won't interfere constructor's initial map.
     Object* new_map;
-    { MaybeObject* maybe_new_map = old_map->CopyDropTransitions();
+    { MaybeObject* maybe_new_map =
+          old_map->CopyDropTransitions(DescriptorArray::MAY_BE_SHARED);
       if (!maybe_new_map->ToObject(&new_map)) return maybe_new_map;
     }
 
@@ -1254,7 +1255,8 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_EnableAccessChecks) {
   if (!old_map->is_access_check_needed()) {
     // Copy map so it won't interfere constructor's initial map.
     Object* new_map;
-    { MaybeObject* maybe_new_map = old_map->CopyDropTransitions();
+    { MaybeObject* maybe_new_map =
+          old_map->CopyDropTransitions(DescriptorArray::MAY_BE_SHARED);
       if (!maybe_new_map->ToObject(&new_map)) return maybe_new_map;
     }
 
@@ -10239,7 +10241,6 @@ static MaybeObject* DebugLookupResultValue(Heap* heap,
     }
     case INTERCEPTOR:
     case MAP_TRANSITION:
-    case ELEMENTS_TRANSITION:
     case CONSTANT_TRANSITION:
     case NULL_DESCRIPTOR:
       return heap->undefined_value();