Do not export natives to runtime via js builtins object.

src/bootstrapper.h

Index: src/bootstrapper.h
diff --git a/src/bootstrapper.h b/src/bootstrapper.h
index 39d9e2dfc4724d9b12c3bf1ac367139f733c9718..2efb20b59f33d911a4a671a64f68f43e35fbca28 100644
--- a/src/bootstrapper.h
+++ b/src/bootstrapper.h
@@ -118,6 +118,10 @@ class Bootstrapper final {
   static bool CompileCodeStubBuiltin(Isolate* isolate, int index);
   static bool InstallCodeStubNatives(Isolate* isolate);
 
+  static void ImportNatives(Isolate* isolate, Handle<JSObject> container);
+  static void ImportExperimentalNatives(Isolate* isolate,
+                                        Handle<JSObject> container);
+
  private:
   Isolate* isolate_;
   typedef int NestingCounterType;