Replace AddProperty by AddNamedProperty to speed up the common case

src/object-observe.js

 // Copyright 2012 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.
 
 "use strict";
 
 // Overview:
 //
 // This file contains all of the routing and accounting for Object.observe.
 // User code will interact with these mechanisms via the Object.observe APIs
@@ skipping 432 matching lines @@
   if (!ObjectInfoHasActiveObservers(objectInfo))
     return;
 
   var hasType = !IS_UNDEFINED(type);
   var newRecord = hasType ?
       { object: ObjectInfoGetObject(objectInfo), type: type } :
       { object: ObjectInfoGetObject(objectInfo) };
 
   for (var prop in changeRecord) {
     if (prop === 'object' || (hasType && prop === 'type')) continue;
-    %AddProperty(newRecord, prop, changeRecord[prop], READ_ONLY + DONT_DELETE);
+    %DefineDataPropertyUnchecked(
+        newRecord, prop, changeRecord[prop], READ_ONLY + DONT_DELETE);
   }
   ObjectFreezeJS(newRecord);
 
   ObjectInfoEnqueueInternalChangeRecord(objectInfo, newRecord);
 }
 
 function ObjectInfoEnqueueInternalChangeRecord(objectInfo, changeRecord) {
   // TODO(rossberg): adjust once there is a story for symbols vs proxies.
   if (IS_SYMBOL(changeRecord.name)) return;
 
@@ skipping 179 matching lines @@
     "observe", ArrayObserve,
     "unobserve", ArrayUnobserve
   ));
   InstallFunctions(notifierPrototype, DONT_ENUM, $Array(
     "notify", ObjectNotifierNotify,
     "performChange", ObjectNotifierPerformChange
   ));
 }
 
 SetupObjectObserve();