Fix clang unused-private-field error.

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 715 matching lines @@
 
   static inline ApiNativeScope* Current() {
     return reinterpret_cast<ApiNativeScope*>(
         Thread::GetThreadLocal(Api::api_native_key_));
   }
 
   Zone* zone() { return zone_.GetZone(); }
 
  private:
   ApiZone zone_;
-  ThreadLocalKey key_;
 };
 
 
 // Api growable arrays use a zone for allocation. The constructor
 // picks the zone from the current isolate if in an isolate
 // environment. When outside an isolate environment it picks the zone
 // from the current native scope.
 template<typename T>
 class ApiGrowableArray : public BaseGrowableArray<T, ValueObject> {
  public:
   explicit ApiGrowableArray(int initial_capacity)
       : BaseGrowableArray<T, ValueObject>(
           initial_capacity,
           ApiNativeScope::Current()->zone()) {}
   ApiGrowableArray()
       : BaseGrowableArray<T, ValueObject>(
           ApiNativeScope::Current()->zone()) {}
 };
 
 
 }  // namespace dart
 
 #endif  // VM_DART_API_STATE_H_