Fix shutdown crash in Sky

sky/engine/tonic/dart_state.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SKY_ENGINE_TONIC_DART_STATE_H_
 #define SKY_ENGINE_TONIC_DART_STATE_H_
 
 #include "base/logging.h"
 #include "base/memory/weak_ptr.h"
 #include "base/supports_user_data.h"
@@ skipping 30 matching lines @@
 
   DartState();
   virtual ~DartState();
 
   static DartState* From(Dart_Isolate isolate);
   static DartState* Current();
 
   base::WeakPtr<DartState> GetWeakPtr();
 
   Dart_Isolate isolate() { return isolate_; }
-  void set_isolate(Dart_Isolate isolate) {
-    CHECK(!isolate_);
-    isolate_ = isolate;
-    DidSetIsolateInternal();
-  }
+  void SetIsolate(Dart_Isolate isolate);
 
   DartClassLibrary& class_library() { return *class_library_; }
   DartStringCache& string_cache() { return *string_cache_; }
   DartExceptionFactory& exception_factory() { return *exception_factory_; }
   DartTimerHeap& timer_heap() { return *timer_heap_; }
 
   Dart_Handle index_handle() { return index_handle_.value(); }
 
   virtual void DidSetIsolate() {}
 
  private:
-  void DidSetIsolateInternal();
-
   Dart_Isolate isolate_;
   OwnPtr<DartClassLibrary> class_library_;
   OwnPtr<DartStringCache> string_cache_;
   OwnPtr<DartExceptionFactory> exception_factory_;
   OwnPtr<DartTimerHeap> timer_heap_;
+  DartPersistentValue index_handle_;
 
+ protected:
   base::WeakPtrFactory<DartState> weak_factory_;
-  DartPersistentValue index_handle_;
 
   DISALLOW_COPY_AND_ASSIGN(DartState);
 };
 
 }  // namespace blink
 
 #endif  // SKY_ENGINE_TONIC_DART_STATE_H_