Make v8::Eternal::Get and IsEmpty const.

include/v8.h

 // Copyright 2012 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.
 
 /** \mainpage V8 API Reference Guide
  *
  * V8 is Google's open source JavaScript engine.
  *
  * This set of documents provides reference material generated from the
  * V8 header file, include/v8.h.
@@ skipping 348 matching lines @@
 
 // Eternal handles are set-once handles that live for the life of the isolate.
 template <class T> class Eternal {
  public:
   V8_INLINE Eternal() : index_(kInitialValue) { }
   template<class S>
   V8_INLINE Eternal(Isolate* isolate, Local<S> handle) : index_(kInitialValue) {
     Set(isolate, handle);
   }
   // Can only be safely called if already set.
-  V8_INLINE Local<T> Get(Isolate* isolate);
-  V8_INLINE bool IsEmpty() { return index_ == kInitialValue; }
+  V8_INLINE Local<T> Get(Isolate* isolate) const;
+  V8_INLINE bool IsEmpty() const { return index_ == kInitialValue; }
   template<class S> V8_INLINE void Set(Isolate* isolate, Local<S> handle);
 
  private:
   static const int kInitialValue = -1;
   int index_;
 };
 
 
 static const int kInternalFieldsInWeakCallback = 2;
 
@@ skipping 8249 matching lines @@
 }
 
 
 template<class T>
 template<class S>
 void Eternal<T>::Set(Isolate* isolate, Local<S> handle) {
   TYPE_CHECK(T, S);
   V8::Eternalize(isolate, reinterpret_cast<Value*>(*handle), &this->index_);
 }
 
-
-template<class T>
-Local<T> Eternal<T>::Get(Isolate* isolate) {
+template <class T>
+Local<T> Eternal<T>::Get(Isolate* isolate) const {
   return Local<T>(reinterpret_cast<T*>(*V8::GetEternal(isolate, index_)));
 }
 
 
 template <class T>
 Local<T> MaybeLocal<T>::ToLocalChecked() {
   if (V8_UNLIKELY(val_ == nullptr)) V8::ToLocalEmpty();
   return Local<T>(val_);
 }
 
@@ skipping 1178 matching lines @@
  */
 
 
 }  // namespace v8
 
 
 #undef TYPE_CHECK
 
 
 #endif  // INCLUDE_V8_H_