[es6] Support optional "receiver" argument in Reflect.get.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 47c9ee929ad3f381d2862e1e7c95d1e8af0a7af1..8fd25b4ce6c2c56989657ef5c5c7ae5fc5f5a80d 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1250,6 +1250,9 @@ class Object {
   MUST_USE_RESULT static inline MaybeHandle<Object> GetPropertyOrElement(
       Handle<Object> object, Handle<Name> name,
       LanguageMode language_mode = SLOPPY);
+  MUST_USE_RESULT static inline MaybeHandle<Object> GetPropertyOrElement(
+      Handle<JSReceiver> holder, Handle<Name> name, Handle<Object> receiver,
+      LanguageMode language_mode = SLOPPY);
   MUST_USE_RESULT static inline MaybeHandle<Object> GetProperty(
       Isolate* isolate, Handle<Object> object, const char* key,
       LanguageMode language_mode = SLOPPY);