Implement immutable prototype chains

test/cctest/test-api.cc

 // 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 25330 matching lines @@
 }
 
 TEST(PrivateForApiIsNumber) {
   LocalContext context;
   v8::Isolate* isolate = CcTest::isolate();
   v8::HandleScope scope(isolate);
 
   // Shouldn't crash.
   v8::Private::ForApi(isolate, v8_str("42"));
 }
+
+THREADED_TEST(ImmutableProto) {
+  LocalContext context;
+  v8::Isolate* isolate = context->GetIsolate();
+  v8::HandleScope handle_scope(isolate);
+
+  Local<v8::FunctionTemplate> templ = v8::FunctionTemplate::New(isolate);
+  templ->InstanceTemplate()->SetImmutableProto();
+
+  Local<v8::Object> object = templ->GetFunction(context.local())
+                                 .ToLocalChecked()
+                                 ->NewInstance(context.local())
+                                 .ToLocalChecked();
+
+  // Look up the prototype
+  Local<v8::Value> original_proto =
+      object->Get(context.local(), v8_str("__proto__")).ToLocalChecked();
+
+  // Setting the prototype (e.g., to null) throws
+  CHECK(object->SetPrototype(context.local(), v8::Null(isolate)).IsNothing());
+
+  // The original prototype is still there
+  Local<Value> new_proto =
+      object->Get(context.local(), v8_str("__proto__")).ToLocalChecked();
+  CHECK(new_proto->IsObject());
+  CHECK(new_proto.As<v8::Object>()
+            ->Equals(context.local(), original_proto)
+            .FromJust());
+}