Object.defineProperty shouldn't be a hint that we're constructing a dictionary.

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 "v8.h"
 
 #include "accessors.h"
 #include "allocation-site-scopes.h"
 #include "api.h"
 #include "arguments.h"
@@ skipping 4300 matching lines @@
 // Note that this method cannot be used to set the prototype of a function
 // because ConvertDescriptorToField() which is called in "case CALLBACKS:"
 // doesn't handle function prototypes correctly.
 MaybeHandle<Object> JSObject::SetLocalPropertyIgnoreAttributes(
     Handle<JSObject> object,
     Handle<Name> name,
     Handle<Object> value,
     PropertyAttributes attributes,
     ValueType value_type,
     StoreMode mode,
-    ExtensibilityCheck extensibility_check) {
+    ExtensibilityCheck extensibility_check,
+    StoreFromKeyed store_from_keyed) {
   Isolate* isolate = object->GetIsolate();
 
   // Make sure that the top context does not change when doing callbacks or
   // interceptor calls.
   AssertNoContextChange ncc(isolate);
 
   LookupResult lookup(isolate);
   object->LocalLookup(name, &lookup, true);
   if (!lookup.IsFound()) {
     object->map()->LookupTransition(*object, *name, &lookup);
@@ skipping 20 matching lines @@
     object->LocalLookupRealNamedProperty(name, &lookup);
   }
 
   // Check for accessor in prototype chain removed here in clone.
   if (!lookup.IsFound()) {
     object->map()->LookupTransition(*object, *name, &lookup);
     TransitionFlag flag = lookup.IsFound()
         ? OMIT_TRANSITION : INSERT_TRANSITION;
     // Neither properties nor transitions found.
     return AddProperty(object, name, value, attributes, SLOPPY,
-        MAY_BE_STORE_FROM_KEYED, extensibility_check, value_type, mode, flag);
+        store_from_keyed, extensibility_check, value_type, mode, flag);
   }
 
   Handle<Object> old_value = isolate->factory()->the_hole_value();
   PropertyAttributes old_attributes = ABSENT;
   bool is_observed = object->map()->is_observed() &&
                      *name != isolate->heap()->hidden_string();
   if (is_observed && lookup.IsProperty()) {
     if (lookup.IsDataProperty()) {
       old_value = Object::GetPropertyOrElement(object, name).ToHandleChecked();
     }
@@ skipping 12844 matching lines @@
 #define ERROR_MESSAGES_TEXTS(C, T) T,
   static const char* error_messages_[] = {
       ERROR_MESSAGES_LIST(ERROR_MESSAGES_TEXTS)
   };
 #undef ERROR_MESSAGES_TEXTS
   return error_messages_[reason];
 }
 
 
 } }  // namespace v8::internal