[es6] Reintroduce the instanceof operator in the backends.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 903512b0312e7a152a00060848407f2f16451052..0bce56c402ab7de3f98b3cf605e86f7524676a0c 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1233,6 +1233,14 @@ class Object {
                                                         Handle<Object> lhs,
                                                         Handle<Object> rhs);
 
+  // ES6 section 7.3.19 OrdinaryHasInstance (C, O).
+  MUST_USE_RESULT static MaybeHandle<Object> OrdinaryHasInstance(
+      Isolate* isolate, Handle<Object> callable, Handle<Object> object);
+
+  // ES6 section 12.10.4 Runtime Semantics: InstanceofOperator(O, C)
+  MUST_USE_RESULT static MaybeHandle<Object> InstanceOf(
+      Isolate* isolate, Handle<Object> object, Handle<Object> callable);
+
   MUST_USE_RESULT static MaybeHandle<Object> GetProperty(LookupIterator* it);
 
   // ES6 [[Set]] (when passed DONT_THROW)