add missing dcheck to ToLocalChecked

R=svenpanne@chromium.org
BUG=v8:3929
LOG=n

Review URL: https://codereview.chromium.org/1017663002

Cr-Commit-Position: refs/heads/master@{#27229}

include/v8.h

@@ -429,10 +429,7 @@ class MaybeLocal {
     return !IsEmpty();
   }
 
-  V8_INLINE Local<T> ToLocalChecked() {
-    // TODO(dcarney): add DCHECK.
-    return Local<T>(val_);
-  }
+  V8_INLINE Local<T> ToLocalChecked();
 
   template <class S>
   V8_INLINE Local<S> FromMaybe(Local<S> default_value) const {
@@ -5893,10 +5890,13 @@ class V8_EXPORT V8 {
   static Local<Value> GetEternal(Isolate* isolate, int index);
 
   static void CheckIsJust(bool is_just);
+  static void ToLocalEmpty();
 
   template <class T> friend class Handle;
   template <class T> friend class Local;
   template <class T>
+  friend class MaybeLocal;
+  template <class T>
   friend class Maybe;
   template <class T> friend class Eternal;
   template <class T> friend class PersistentBase;
@@ -6725,6 +6725,15 @@ Local<T> Eternal<T>::Get(Isolate* isolate) {
 }
 
 
+template <class T>
+Local<T> MaybeLocal<T>::ToLocalChecked() {
+#ifdef V8_ENABLE_CHECKS
+  if (val_ == nullptr) V8::ToLocalEmpty();
+#endif
+  return Local<T>(val_);
+}
+
+
 template <class T>
 T* PersistentBase<T>::New(Isolate* isolate, T* that) {
   if (that == NULL) return NULL;

src/api.cc

@@ -617,6 +617,11 @@ void V8::CheckIsJust(bool is_just) {
 }
 
 
+void V8::ToLocalEmpty() {
+  Utils::ApiCheck(false, "v8::ToLocalChecked", "Empty MaybeLocal.");
+}
+
+
 // --- H a n d l e s ---