[runtime] Don't crash when creating an instance of a class inherited from a Proxy.

src/objects.cc

 // Copyright 2015 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 13146 matching lines @@
 
 
 void JSFunction::CalculateInstanceSizeForDerivedClass(
     InstanceType instance_type, int requested_internal_fields,
     int* instance_size, int* in_object_properties) {
   Isolate* isolate = GetIsolate();
   int expected_nof_properties = 0;
   for (PrototypeIterator iter(isolate, this,
                               PrototypeIterator::START_AT_RECEIVER);
        !iter.IsAtEnd(); iter.Advance()) {
-    JSFunction* func = iter.GetCurrent<JSFunction>();
+    JSReceiver* current = iter.GetCurrent<JSReceiver>();
+    if (!current->IsJSFunction()) break;
+    JSFunction* func = JSFunction::cast(current);
     SharedFunctionInfo* shared = func->shared();
     expected_nof_properties += shared->expected_nof_properties();
     if (!IsSubclassConstructor(shared->kind())) {
       break;
     }
   }
   CalculateInstanceSizeHelper(instance_type, requested_internal_fields,
                               expected_nof_properties, instance_size,
                               in_object_properties);
 }
@@ skipping 5709 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