Don't roundtrip to JS to update the length. This is 1) faster (although we don't care) and 2) avoid…

src/objects.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <iomanip>
 #include <sstream>
 
 #include "src/v8.h"
 
 #include "src/accessors.h"
@@ skipping 12000 matching lines @@
   Handle<JSArray> deleted = isolate->factory()->NewJSArray(0);
   if (delete_count > 0) {
     for (int i = indices.length() - 1; i >= 0; i--) {
       // Skip deletions where the property was an accessor, leaving holes
       // in the array of old values.
       if (old_values[i]->IsTheHole()) continue;
       JSObject::AddDataElement(deleted, indices[i] - index, old_values[i], NONE)
           .Assert();
     }
 
-    SetProperty(deleted, isolate->factory()->length_string(),
-                isolate->factory()->NewNumberFromUint(delete_count),
-                STRICT).Assert();
+    ElementsAccessor* accessor = deleted->GetElementsAccessor();
+    accessor->SetLength(deleted, isolate->factory()->NewNumberFromUint(
+                                     delete_count)).Check();
   }
 
   RETURN_ON_EXCEPTION(
       isolate, EnqueueSpliceRecord(array, index, deleted, add_count), Object);
 
   return hresult;
 }
 
 
 // static
@@ skipping 4640 matching lines @@
                                             Handle<Object> new_value) {
   if (cell->value() != *new_value) {
     cell->set_value(*new_value);
     Isolate* isolate = cell->GetIsolate();
     cell->dependent_code()->DeoptimizeDependentCodeGroup(
         isolate, DependentCode::kPropertyCellChangedGroup);
   }
 }
 }  // namespace internal
 }  // namespace v8