[es6] Reintroduce the instanceof operator in the backends.

src/lookup.h

Index: src/lookup.h
diff --git a/src/lookup.h b/src/lookup.h
index 94a3f2c017b2c55592a9c4a1690ff9c604d8f1c1..bb65639165ad23324f5cee1668d689c72eb860c8 100644
--- a/src/lookup.h
+++ b/src/lookup.h
@@ -263,7 +263,9 @@ class LookupIterator final BASE_EMBEDDED {
     if (IsElement()) return;
     if (*name_ == heap()->is_concat_spreadable_symbol() ||
         (FLAG_harmony_species && (*name_ == heap()->constructor_string() ||
-                                  *name_ == heap()->species_symbol()))) {
+                                  *name_ == heap()->species_symbol())) ||
+        (FLAG_harmony_instanceof &&
+         (*name_ == heap()->has_instance_symbol()))) {
       InternalUpdateProtector();
     }
   }