The Elements pointer in a JSObject can have a filler map instead of a

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 006aff394c20cd517c516e0823424f3aa42c9c80..7b2ac766418d7e4dce84ee3d1d6edb2da2d75501 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -5516,19 +5516,24 @@ ElementsKind JSObject::GetElementsKind() {
 #if DEBUG
   FixedArrayBase* fixed_array =
       reinterpret_cast<FixedArrayBase*>(READ_FIELD(this, kElementsOffset));
-  Map* map = fixed_array->map();
-  ASSERT((IsFastSmiOrObjectElementsKind(kind) &&
-          (map == GetHeap()->fixed_array_map() ||
-           map == GetHeap()->fixed_cow_array_map())) ||
-         (IsFastDoubleElementsKind(kind) &&
-          (fixed_array->IsFixedDoubleArray() ||
-           fixed_array == GetHeap()->empty_fixed_array())) ||
-         (kind == DICTIONARY_ELEMENTS &&
+
+  // If a GC was caused while constructing this object, the elements
+  // pointer may point to a one pointer filler map.
+  if (ElementsAreSafeToExamine()) {
+    Map* map = fixed_array->map();
+    ASSERT((IsFastSmiOrObjectElementsKind(kind) &&
+            (map == GetHeap()->fixed_array_map() ||
+             map == GetHeap()->fixed_cow_array_map())) ||
+           (IsFastDoubleElementsKind(kind) &&
+            (fixed_array->IsFixedDoubleArray() ||
+             fixed_array == GetHeap()->empty_fixed_array())) ||
+           (kind == DICTIONARY_ELEMENTS &&
             fixed_array->IsFixedArray() &&
-          fixed_array->IsDictionary()) ||
-         (kind > DICTIONARY_ELEMENTS));
-  ASSERT((kind != NON_STRICT_ARGUMENTS_ELEMENTS) ||
-         (elements()->IsFixedArray() && elements()->length() >= 2));
+            fixed_array->IsDictionary()) ||
+           (kind > DICTIONARY_ELEMENTS));
+    ASSERT((kind != NON_STRICT_ARGUMENTS_ELEMENTS) ||
+           (elements()->IsFixedArray() && elements()->length() >= 2));
+  }
 #endif
   return kind;
 }