Fix performance regression introduced in r28961

src/runtime/runtime-function.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/accessors.h"
 #include "src/arguments.h"
 #include "src/compiler.h"
 #include "src/cpu-profiler.h"
@@ skipping 429 matching lines @@
   new_bindings->set_map_no_write_barrier(isolate->heap()->fixed_array_map());
   bound_function->set_function_bindings(*new_bindings);
 
   // Update length. Have to remove the prototype first so that map migration
   // is happy about the number of fields.
   RUNTIME_ASSERT(bound_function->RemovePrototype());
   Handle<Map> bound_function_map(
       isolate->native_context()->bound_function_map());
   JSObject::MigrateToMap(bound_function, bound_function_map);
   Handle<String> length_string = isolate->factory()->length_string();
+  // These attributes must be kept in sync with how the bootstrapper
+  // configures the bound_function_map retrieved above.
+  // We use ...IgnoreAttributes() here because of length's read-onliness.
   PropertyAttributes attr =
       static_cast<PropertyAttributes>(DONT_ENUM | READ_ONLY);
   RETURN_FAILURE_ON_EXCEPTION(
       isolate, JSObject::SetOwnPropertyIgnoreAttributes(
                    bound_function, length_string, new_length, attr));
   return *bound_function;
 }
 
 
 RUNTIME_FUNCTION(Runtime_BoundFunctionGetBindings) {
@@ skipping 165 matching lines @@
 
 
 RUNTIME_FUNCTION(Runtime_ThrowStrongModeTooFewArguments) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 0);
   THROW_NEW_ERROR_RETURN_FAILURE(isolate,
                                  NewTypeError(MessageTemplate::kStrongArity));
 }
 }  // namespace internal
 }  // namespace v8