Add GetProperty/GetElement to JSReceiver and use it where possible

src/contexts.cc

Index: src/contexts.cc
diff --git a/src/contexts.cc b/src/contexts.cc
index 517d7ecc47cffde56d8d10975a3a3917064ccab5..9adccb2897778611d6a89556561218371791175e 100644
--- a/src/contexts.cc
+++ b/src/contexts.cc
@@ -164,14 +164,16 @@ static Maybe<bool> UnscopableLookup(LookupIterator* it) {
   Handle<Object> unscopables;
   ASSIGN_RETURN_ON_EXCEPTION_VALUE(
       isolate, unscopables,
-      Object::GetProperty(it->GetReceiver(),
-                          isolate->factory()->unscopables_symbol()),
+      JSReceiver::GetProperty(Handle<JSReceiver>::cast(it->GetReceiver()),
+                              isolate->factory()->unscopables_symbol()),
       Nothing<bool>());
   if (!unscopables->IsJSReceiver()) return Just(true);
   Handle<Object> blacklist;
-  ASSIGN_RETURN_ON_EXCEPTION_VALUE(isolate, blacklist,
-                                   Object::GetProperty(unscopables, it->name()),
-                                   Nothing<bool>());
+  ASSIGN_RETURN_ON_EXCEPTION_VALUE(
+      isolate, blacklist,
+      JSReceiver::GetProperty(Handle<JSReceiver>::cast(unscopables),
+                              it->name()),
+      Nothing<bool>());
   return Just(!blacklist->BooleanValue());
 }