[tests] Fix bogus uses of assertThrows.

test/mjsunit/es6/object-tostring.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.
 
 // Flags: --harmony-tostring --harmony-proxies
 
 var global = this;
 
 var funs = {
   Object:   [ Object ],
@@ skipping 23 matching lines @@
   var obj = {};
   obj[Symbol.toStringTag] = className;
   assertEquals("[object " + className + "]",
                Object.prototype.toString.call(obj));
 
   // Getter throws
   obj = {};
   Object.defineProperty(obj, Symbol.toStringTag, {
     get: function() { throw className; }
   });
-  assertThrows(function() {
+  assertThrowsEquals(function() {
     Object.prototype.toString.call(obj);
   }, className);
 
   // Getter does not throw
   obj = {};
   Object.defineProperty(obj, Symbol.toStringTag, {
     get: function() { return className; }
   });
   assertEquals("[object " + className + "]",
                Object.prototype.toString.call(obj));
@@ skipping 97 matching lines @@
 assertTag("Function", new Proxy(() => 42, {get() {return 666}}));
 
 revocable = Proxy.revocable([], {});
 revocable.revoke();
 assertThrows(() => Object.prototype.toString.call(revocable.proxy), TypeError);
 
 handler = {};
 revocable = Proxy.revocable([], handler);
 handler.get = () => revocable.revoke();
 assertThrows(() => Object.prototype.toString.call(revocable.proxy), TypeError);