[es6] Function bind should preserve [[Prototype]]

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 425 matching lines @@
   // Copy arguments, skipping the first which is "this_arg".
   for (int j = 0; j < argc; j++, i++) {
     new_bindings->set(i, *arguments[j + 1]);
   }
   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());
+
+  // The new function should have the same [[Prototype]] as the bindee.
   Handle<Map> bound_function_map(
       isolate->native_context()->bound_function_map());
+  PrototypeIterator iter(isolate, bindee);
+  Handle<Object> proto = PrototypeIterator::GetCurrent(iter);
+  if (bound_function_map->prototype() != *proto) {
+    bound_function_map = Map::TransitionToPrototype(bound_function_map, proto,
+                                                    REGULAR_PROTOTYPE);
+  }
   JSObject::MigrateToMap(bound_function, bound_function_map);
+
   Handle<String> length_string = isolate->factory()->length_string();
   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;
 }
 
 
@@ skipping 166 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