[es6] Reintroduce the instanceof operator in the backends.

test/cctest/test-asm-validator.cc

Index: test/cctest/test-asm-validator.cc
diff --git a/test/cctest/test-asm-validator.cc b/test/cctest/test-asm-validator.cc
index 3902c9f06fe1b109248b86c1608aa6abf0b76207..a07eecf2760644d9309eaf7034583bb1c3d4b2ac 100644
--- a/test/cctest/test-asm-validator.cc
+++ b/test/cctest/test-asm-validator.cc
@@ -1800,14 +1800,10 @@ TEST(NeStrict) {
 
 
 TEST(InstanceOf) {
-  const char* errorMsg = FLAG_harmony_instanceof
-                             ? "asm: line 0: do-expression encountered\n"
-                             : "asm: line 1: illegal comparison operator\n";
-
   CHECK_FUNC_ERROR(
       "function bar() { return (0 instanceof 0)|0; }\n"
       "function foo() { bar(); }",
-      errorMsg);
+      "asm: line 1: illegal comparison operator\n");
 }