Use JSON.stringify to serialize debugger messages

test/mjsunit/mirror-string.js

 // Copyright 2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 16 matching lines @@
 
 // Flags: --expose-debug-as debug
 // Test the mirror object for string values
 
 const kMaxProtocolStringLength = 80; // Constant from mirror-delay.js
 
 function testStringMirror(s) {
   // Create mirror and JSON representation.
   var mirror = debug.MakeMirror(s);
   var serializer = debug.MakeMirrorSerializer();
-  var json = serializer.serializeValue(mirror);
+  var json = JSON.stringify(serializer.serializeValue(mirror));
 
   // Check the mirror hierachy.
   assertTrue(mirror instanceof debug.Mirror);
   assertTrue(mirror instanceof debug.ValueMirror);
   assertTrue(mirror instanceof debug.StringMirror);
 
   // Check the mirror properties.
   assertTrue(mirror.isString());
   assertEquals('string', mirror.type());
   assertTrue(mirror.isPrimitive());
@@ skipping 32 matching lines @@
 testStringMirror('\b\t\n\f\r');
 testStringMirror('\u0001\u0002\u001E\u001F');
 testStringMirror('"a":1,"b":2');
 
 var s = "1234567890"
 s = s + s + s + s + s + s + s + s;
 assertEquals(kMaxProtocolStringLength, s.length);
 testStringMirror(s);
 s = s + 'X';
 testStringMirror(s);