Fix Mac build.

runtime/vm/dart_api_state.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_DART_API_STATE_H_
 #define VM_DART_API_STATE_H_
 
 #include "include/dart_api.h"
 
 #include "platform/thread.h"
@@ skipping 608 matching lines @@
 class ApiNativeScope {
  public:
   ApiNativeScope() {
     // Currently no support for nesting native scopes.
     ASSERT(Current() == NULL);
     Thread::SetThreadLocal(Api::api_native_key_, reinterpret_cast<uword>(this));
   }
 
   ~ApiNativeScope() {
     ASSERT(Current() == this);
-    Thread::SetThreadLocal(Api::api_native_key_, NULL);
+    Thread::SetThreadLocal(Api::api_native_key_, 0);
   }
 
   static inline ApiNativeScope* Current() {
     return reinterpret_cast<ApiNativeScope*>(
         Thread::GetThreadLocal(Api::api_native_key_));
   }
 
   ApiZone* zone() { return &zone_; }
 
  private:
@@ skipping 15 matching lines @@
           ApiNativeScope::Current()->zone()->GetBaseZone()) {}
   ApiGrowableArray()
       : BaseGrowableArray<T, ValueObject>(
           ApiNativeScope::Current()->zone()->GetBaseZone()) {}
 };
 
 
 }  // namespace dart
 
 #endif  // VM_DART_API_STATE_H_