Fix JSON encoder accepting NaN and Infinity.

tests/lib/convert/json_lib_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library json_tests;
 import 'package:unittest/unittest.dart';
 import 'dart:convert';
 
 main() {
   test('Parse', () {
@@ skipping 169 matching lines @@
 var throwsJsonError =
     throwsA(new isInstanceOf<JsonUnsupportedObjectError>());
 
 /**
  * Checks that the argument can be converted to a JSON string and
  * back, and produce something equivalent to the argument.
  */
 validateRoundTrip(expected) {
   expect(JSON.decode(JSON.encode(expected)), equals(expected));
 }