Handle setters on the prototype chain efficiently by default.

src/ic.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1298 matching lines @@
   }
   return !lookup->IsReadOnly();
 }
 
 
 static bool LookupForWrite(Handle<JSObject> receiver,
                            Handle<String> name,
                            LookupResult* lookup) {
   receiver->LocalLookup(*name, lookup);
   if (!StoreICableLookup(lookup)) {
-    // 2nd chance: There can be accessors somewhere in the prototype chain, but
-    // for compatibility reasons we have to hide this behind a flag. Note that
-    // we explicitly exclude native accessors for now, because the stubs are not
-    // yet prepared for this scenario.
-    if (!FLAG_es5_readonly) return false;
+    // 2nd chance: There can be accessors somewhere in the prototype chain. Note
+    // that we explicitly exclude native accessors for now, because the stubs
+    // are not yet prepared for this scenario.
     receiver->Lookup(*name, lookup);
     if (!lookup->IsCallbacks()) return false;
     Handle<Object> callback(lookup->GetCallbackObject());
     return callback->IsAccessorPair() && StoreICableLookup(lookup);
   }
 
   if (lookup->IsInterceptor() &&
       receiver->GetNamedInterceptor()->setter()->IsUndefined()) {
     receiver->LocalLookupRealNamedProperty(*name, lookup);
     return StoreICableLookup(lookup);
@@ skipping 1369 matching lines @@
 #undef ADDR
 };
 
 
 Address IC::AddressFromUtilityId(IC::UtilityId id) {
   return IC_utilities[id];
 }
 
 
 } }  // namespace v8::internal