[es6] Fix WeakMap/Set built-ins subclasssing.

src/objects.cc

 // Copyright 2013 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/objects.h"
 
 #include <cmath>
 #include <iomanip>
 #include <sstream>
 
@@ skipping 17313 matching lines @@
 
 void JSMap::Clear(Handle<JSMap> map) {
   Handle<OrderedHashMap> table(OrderedHashMap::cast(map->table()));
   table = OrderedHashMap::Clear(table);
   map->set_table(*table);
 }
 
 
 void JSWeakCollection::Initialize(Handle<JSWeakCollection> weak_collection,
                                   Isolate* isolate) {
-  DCHECK_EQ(0, weak_collection->map()->GetInObjectProperties());
   Handle<ObjectHashTable> table = ObjectHashTable::New(isolate, 0);
   weak_collection->set_table(*table);
 }
 
 
 void JSWeakCollection::Set(Handle<JSWeakCollection> weak_collection,
                            Handle<Object> key, Handle<Object> value,
                            int32_t hash) {
   DCHECK(key->IsJSReceiver() || key->IsSymbol());
   Handle<ObjectHashTable> table(
@@ skipping 682 matching lines @@
   if (cell->value() != *new_value) {
     cell->set_value(*new_value);
     Isolate* isolate = cell->GetIsolate();
     cell->dependent_code()->DeoptimizeDependentCodeGroup(
         isolate, DependentCode::kPropertyCellChangedGroup);
   }
 }
 
 }  // namespace internal
 }  // namespace v8