Avoid some unnecessary fast-properties map creations.

src/runtime/runtime-object.cc

 // Copyright 2014 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 "src/v8.h"
 
 #include "src/arguments.h"
 #include "src/bootstrapper.h"
 #include "src/debug.h"
 #include "src/runtime/runtime.h"
@@ skipping 1435 matching lines @@
   RUNTIME_ASSERT(!obj->IsNull());
   CONVERT_ARG_HANDLE_CHECKED(Name, name, 1);
   CONVERT_ARG_HANDLE_CHECKED(Object, getter, 2);
   RUNTIME_ASSERT(IsValidAccessor(getter));
   CONVERT_ARG_HANDLE_CHECKED(Object, setter, 3);
   RUNTIME_ASSERT(IsValidAccessor(setter));
   CONVERT_SMI_ARG_CHECKED(unchecked, 4);
   RUNTIME_ASSERT((unchecked & ~(READ_ONLY | DONT_ENUM | DONT_DELETE)) == 0);
   PropertyAttributes attr = static_cast<PropertyAttributes>(unchecked);
 
-  bool fast = obj->HasFastProperties();
   RETURN_FAILURE_ON_EXCEPTION(
       isolate, JSObject::DefineAccessor(obj, name, getter, setter, attr));
-  if (fast) JSObject::MigrateSlowToFast(obj, 0);
   return isolate->heap()->undefined_value();
 }
 
 
 // Implements part of 8.12.9 DefineOwnProperty.
 // There are 3 cases that lead here:
 // Step 4a - define a new data property.
 // Steps 9b & 12 - replace an existing accessor property with a data property.
 // Step 12 - update an existing data property with a data or generic
 //           descriptor.
@@ skipping 108 matching lines @@
 
 RUNTIME_FUNCTION(RuntimeReference_ClassOf) {
   SealHandleScope shs(isolate);
   DCHECK(args.length() == 1);
   CONVERT_ARG_CHECKED(Object, obj, 0);
   if (!obj->IsJSReceiver()) return isolate->heap()->null_value();
   return JSReceiver::cast(obj)->class_name();
 }
 }
 }  // namespace v8::internal