[es6] Reintroduce the instanceof operator in the backends.

test/cctest/interpreter/test-interpreter.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/execution.h"
 #include "src/handles.h"
 #include "src/interpreter/bytecode-array-builder.h"
 #include "src/interpreter/bytecode-array-iterator.h"
@@ skipping 1365 matching lines @@
       CHECK(return_value->IsBoolean());
       CHECK_EQ(return_value->BooleanValue(),
                CompareC(Token::Value::NE_STRICT, inputs_number[i],
                         inputs_number[j]));
     }
   }
 }
 
 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);
   Handle<i::JSObject> instance = factory->NewJSObject(func);
   Handle<i::Object> other = factory->NewNumber(3.3333);
   Handle<i::Object> cases[] = {Handle<i::Object>::cast(instance), other};
   for (size_t i = 0; i < arraysize(cases); i++) {
     bool expected_value = (i == 0);
     BytecodeArrayBuilder builder(handles.main_isolate(), handles.main_zone(), 0,
                                  0, 1);
@@ skipping 2767 matching lines @@
     CHECK(return_value->SameValue(*tests[i].second));
   }
 
   FLAG_ignition_generators = old_flag;
 }
 
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8