Use JSON.stringify to serialize debugger messages

test/mjsunit/mirror-undefined.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 13 matching lines @@
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 // Flags: --expose-debug-as debug
 // Test the mirror object for undefined
 
 // Create mirror and JSON representation.
 var mirror = debug.MakeMirror(void 0);
 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.UndefinedMirror);
 
 // Check the mirror properties.
 assertTrue(mirror.isUndefined());
 assertEquals('undefined', mirror.type());
 assertTrue(mirror.isPrimitive());
 
 // Test text representation
 assertEquals('undefined', mirror.toText());
 
 // Parse JSON representation and check.
 var fromJSON = eval('(' + json + ')');
 assertEquals('undefined', fromJSON.type);