Fix warning in build

runtime/vm/object.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_OBJECT_H_
 #define VM_OBJECT_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/utils.h"
@@ skipping 686 matching lines @@
   void operator=(RawObject* value) {
     raw_ = value;
   }
   void operator^=(RawObject* value) {
     raw_ = value;
   }
   static PassiveObject& Handle(Isolate* I, RawObject* raw_ptr) {
     PassiveObject* obj =
         reinterpret_cast<PassiveObject*>(VMHandles::AllocateHandle(I));
     obj->raw_ = raw_ptr;
-    obj->set_vtable(NULL);
+    obj->set_vtable(0);
     return *obj;
   }
   static PassiveObject& Handle(RawObject* raw_ptr) {
     return Handle(Isolate::Current(), raw_ptr);
   }
   static PassiveObject& Handle() {
     return Handle(Isolate::Current(), Object::null());
   }
   static PassiveObject& Handle(Isolate* I) {
     return Handle(I, Object::null());
   }
   static PassiveObject& ZoneHandle(Isolate* I, RawObject* raw_ptr) {
     PassiveObject* obj = reinterpret_cast<PassiveObject*>(
         VMHandles::AllocateZoneHandle(I));
     obj->raw_ = raw_ptr;
-    obj->set_vtable(NULL);
+    obj->set_vtable(0);
     return *obj;
   }
   static PassiveObject& ZoneHandle(RawObject* raw_ptr) {
     return ZoneHandle(Isolate::Current(), raw_ptr);
   }
   static PassiveObject& ZoneHandle() {
     return ZoneHandle(Isolate::Current(), Object::null());
   }
   static PassiveObject& ZoneHandle(Isolate* I) {
     return ZoneHandle(I, Object::null());
@@ skipping 6675 matching lines @@
 
 
 RawObject* MegamorphicCache::GetTargetFunction(const Array& array,
                                                intptr_t index) {
   return array.At((index * kEntryLength) + kTargetFunctionIndex);
 }
 
 }  // namespace dart
 
 #endif  // VM_OBJECT_H_