[es6] don't "replace" Object.prototype.toString for --harmony-tostring

test/mjsunit/es6/prototype-ordinary-objects.js

 // 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.
 
 
 var funcs = [
 
   // https://code.google.com/p/v8/issues/detail?id=4002
   // Error,
   // EvalError,
@@ skipping 30 matching lines @@
   Uint16Array,
   Uint32Array,
   Uint8Array,
   Uint8ClampedArray,
   WeakMap,
   WeakSet,
 ];
 
 for (var fun of funcs) {
   var p = fun.prototype;
+
+  // @@toStringTag is tested separately, and interferes with this test.
+  if (Symbol.toStringTag) delete p[Symbol.toStringTag];
   assertEquals('[object Object]', Object.prototype.toString.call(p));
 }
 
 
 // These still have special prototypes for legacy reason.
 var funcs = [
   Array,
   Function,
 ];
 
 for (var fun of funcs) {
   var p = fun.prototype;
-  assertEquals('[object ' + fun.name + ']', Object.prototype.toString.call(p));
+  assertEquals(`[object ${fun.name}]`, Object.prototype.toString.call(p));
 }