[es6] Reintroduce the instanceof operator in the backends.

test/cctest/interpreter/test-interpreter.cc

Index: test/cctest/interpreter/test-interpreter.cc
diff --git a/test/cctest/interpreter/test-interpreter.cc b/test/cctest/interpreter/test-interpreter.cc
index d0ebd5e6ca84efe4ea005004d3849808fd5964bb..acc93c5282da1f3a0105eccdf508ff474bc60e21 100644
--- a/test/cctest/interpreter/test-interpreter.cc
+++ b/test/cctest/interpreter/test-interpreter.cc
@@ -1383,11 +1383,6 @@ TEST(InterpreterStrictNotEqual) {
 
 TEST(InterpreterInstanceOf) {
   HandleAndZoneScope handles;
-  // TODO(4447): The new ES6 'instanceof' operator is fully desugared in the
-  // parser and the Token::INSTANCEOF is not needed anymore. This test only
-  // makes sense with --no-harmony-instanceof and can be removed once we
-  // deprecate the ability to switch to old skool ES5 'instanceof' for good.
-  FLAG_harmony_instanceof = false;
   i::Factory* factory = handles.main_isolate()->factory();
   Handle<i::String> name = factory->NewStringFromAsciiChecked("cons");
   Handle<i::JSFunction> func = factory->NewFunction(name);