[es6] Reintroduce the instanceof operator in the backends.

src/parsing/preparser.h

Index: src/parsing/preparser.h
diff --git a/src/parsing/preparser.h b/src/parsing/preparser.h
index 4a818c930d21d4f0eec588f4c0ec1536a4e2fb2c..16eeab440ad1042456913a2e14a1f1979c2017cf 100644
--- a/src/parsing/preparser.h
+++ b/src/parsing/preparser.h
@@ -992,9 +992,6 @@ class PreParserTraits {
 
   inline PreParserExpression RewriteYieldStar(
       PreParserExpression generator, PreParserExpression expr, int pos);
-  inline PreParserExpression RewriteInstanceof(PreParserExpression lhs,
-                                               PreParserExpression rhs,
-                                               int pos);
 
  private:
   PreParser* pre_parser_;
@@ -1232,12 +1229,6 @@ PreParserExpression PreParserTraits::RewriteYieldStar(
   return PreParserExpression::Default();
 }
 
-PreParserExpression PreParserTraits::RewriteInstanceof(PreParserExpression lhs,
-                                                       PreParserExpression rhs,
-                                                       int pos) {
-  return PreParserExpression::Default();
-}
-
 PreParserStatementList PreParser::ParseEagerFunctionBody(
     PreParserIdentifier function_name, int pos,
     const PreParserFormalParameters& parameters, FunctionKind kind,