Add Persistent::Upcast.

include/v8.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1451 matching lines @@
   bool BooleanValue() const;
   double NumberValue() const;
   int64_t IntegerValue() const;
   uint32_t Uint32Value() const;
   int32_t Int32Value() const;
 
   /** JS == */
   bool Equals(Handle<Value> that) const;
   bool StrictEquals(Handle<Value> that) const;
 
+  template <class T> V8_INLINE(static Value* Cast(T* value));
+
  private:
   V8_INLINE(bool QuickIsUndefined() const);
   V8_INLINE(bool QuickIsNull() const);
   V8_INLINE(bool QuickIsString() const);
   bool FullIsUndefined() const;
   bool FullIsNull() const;
   bool FullIsString() const;
 };
 
 
@@ skipping 4498 matching lines @@
 
 bool Value::QuickIsString() const {
   typedef internal::Object O;
   typedef internal::Internals I;
   O* obj = *reinterpret_cast<O**>(const_cast<Value*>(this));
   if (!I::HasHeapObjectTag(obj)) return false;
   return (I::GetInstanceType(obj) < I::kFirstNonstringType);
 }
 
 
+template <class T> Value* Value::Cast(T* value) {
+  return static_cast<Value*>(value);
+}
+
+
 Symbol* Symbol::Cast(v8::Value* value) {
 #ifdef V8_ENABLE_CHECKS
   CheckCast(value);
 #endif
   return static_cast<Symbol*>(value);
 }
 
 
 Number* Number::Cast(v8::Value* value) {
 #ifdef V8_ENABLE_CHECKS
@@ skipping 284 matching lines @@
 
 
 }  // namespace v8
 
 
 #undef V8EXPORT
 #undef TYPE_CHECK
 
 
 #endif  // V8_H_