Give v8::Eternal a direct reference to the handle.

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index fe38d236a481af4e003809437cf3ee1689eff72a..2f3c77600745b4b20482617f32995d3b20569557 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -360,19 +360,18 @@ class MaybeLocal {
 // 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) {
+  V8_INLINE Eternal() : val_(nullptr) {}
+  template <class S>
+  V8_INLINE Eternal(Isolate* isolate, Local<S> handle) : val_(nullptr) {
     Set(isolate, handle);
   }
   // Can only be safely called if already set.
   V8_INLINE Local<T> Get(Isolate* isolate) const;
-  V8_INLINE bool IsEmpty() const { return index_ == kInitialValue; }
+  V8_INLINE bool IsEmpty() const { return val_ == nullptr; }
   template<class S> V8_INLINE void Set(Isolate* isolate, Local<S> handle);
 
  private:
-  static const int kInitialValue = -1;
-  int index_;
+  T* val_;
 };
 
 
@@ -7673,10 +7672,7 @@ class V8_EXPORT V8 {
                        WeakCallbackInfo<void>::Callback weak_callback);
   static void MakeWeak(internal::Object*** location_addr);
   static void* ClearWeak(internal::Object** location);
-  static void Eternalize(Isolate* isolate,
-                         Value* handle,
-                         int* index);
-  static Local<Value> GetEternal(Isolate* isolate, int index);
+  static Value* Eternalize(Isolate* isolate, Value* handle);
 
   static void RegisterExternallyReferencedObject(internal::Object** object,
                                                  internal::Isolate* isolate);
@@ -8641,12 +8637,15 @@ 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_);
+  val_ = reinterpret_cast<T*>(
+      V8::Eternalize(isolate, reinterpret_cast<Value*>(*handle)));
 }
 
 template <class T>
 Local<T> Eternal<T>::Get(Isolate* isolate) const {
-  return Local<T>(reinterpret_cast<T*>(*V8::GetEternal(isolate, index_)));
+  // The eternal handle will never go away, so as with the roots, we don't even
+  // need to open a handle.
+  return Local<T>(val_);
 }