src: fix persistent handle lifecycle issue

Commit 636ca7c adds an optimization that casts strong Persistent<T>
handles directly to Local<T> handles to avoid the overhead of creating
new HandleScope-rooted Local<T> handles all the time.

One gotcha that I missed is that it's no longer legal to reference the
Local<T> after calling Persistent<T>::Dispose(). This commit addresses
that.

src/cares_wrap.cc

@@ -744,7 +744,7 @@ static void Query(const FunctionCallbackInfo<Value>& args) {
   // We must cache the wrap's js object here, because cares might make the
   // callback from the wrap->Send stack. This will destroy the wrap's internal
   // object reference, causing wrap->object() to return an empty handle.
-  Local<Object> object = wrap->object();
+  Local<Object> object = Local<Object>::New(node_isolate, wrap->persistent());
 
   String::Utf8Value name(args[0]);
 
@@ -772,7 +772,7 @@ static void QueryWithFamily(const FunctionCallbackInfo<Value>& args) {
   // We must cache the wrap's js object here, because cares might make the
   // callback from the wrap->Send stack. This will destroy the wrap's internal
   // object reference, causing wrap->object() to return an empty handle.
-  Local<Object> object = wrap->object();
+  Local<Object> object = Local<Object>::New(node_isolate, wrap->persistent());
 
   String::Utf8Value name(args[0]);
   int family = args[1]->Int32Value();

src/node_crypto.cc

@@ -3248,7 +3248,7 @@ void EIO_PBKDF2After(uv_work_t* work_req, int status) {
   assert(status == 0);
   pbkdf2_req* req = container_of(work_req, pbkdf2_req, work_req);
   HandleScope scope(node_isolate);
-  Local<Object> obj = PersistentToLocal(req->obj);
+  Local<Object> obj = Local<Object>::New(node_isolate, req->obj);
   req->obj.Dispose();
   Local<Value> argv[2];
   EIO_PBKDF2After(req, argv);

src/node_internals.h

@@ -62,10 +62,16 @@ public:
   void operator=(v8::Handle<v8::Value> that);
 };
 
+// If persistent.IsWeak() == false, then do not call persistent.Dispose()
+// while the returned Local<T> is still in scope, it will destroy the
+// reference to the object.
 template <class TypeName>
 inline v8::Local<TypeName> PersistentToLocal(
     const v8::Persistent<TypeName>& persistent);
 
+// If persistent.IsWeak() == false, then do not call persistent.Dispose()
+// while the returned Local<T> is still in scope, it will destroy the
+// reference to the object.
 template <class TypeName>
 inline v8::Local<TypeName> PersistentToLocal(
     v8::Isolate* isolate,

src/node_script.cc

@@ -174,7 +174,7 @@ Local<Object> WrappedContext::NewInstance() {
 
 
 Local<Context> WrappedContext::GetV8Context() {
-  return PersistentToLocal(context_);
+  return Local<Context>::New(node_isolate, context_);
 }
 
 
@@ -405,7 +405,7 @@ void WrappedScript::EvalMachine(const FunctionCallbackInfo<Value>& args) {
           "'this' must be a result of previous new Script(code) call.");
     }
 
-    script = PersistentToLocal(n_script->script_);
+    script = Local<Script>::New(node_isolate, n_script->script_);
   }
 
   if (output_flag == returnResult) {