Fix JSON encoder accepting NaN and Infinity.

pkg/json/test/json_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 'package:json/json.dart' as json;
 
 main() {
   test('Parse', () {
@@ skipping 105 matching lines @@
            '[4,{"x":42}]');
 
     expect(() {
       json.stringify([new ToJson(new ToJson(4))]);
     }, throwsJsonError);
 
     expect(() {
       json.stringify([new Object()]);
     }, throwsJsonError);
 
+    expect(() {
+      json.stringify([double.NAN]);
+    }, throwsJsonError);
+
+    expect(() {
+      json.stringify([double.INFINITY]);
+    }, throwsJsonError);
+
+    expect(() {
+      json.stringify([-double.INFINITY]);
+    }, throwsJsonError);
   });
 
   test('stringify throws if argument cannot be converted', () {
     /**
      * Checks that we get an exception (rather than silently returning null) if
      * we try to stringify something that cannot be converted to json.
      */
     expect(() => json.stringify(new TestClass()), throwsJsonError);
   });
 
@@ skipping 44 matching lines @@
 var throwsJsonError =
     throwsA(new isInstanceOf<json.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.parse(json.stringify(expected)), equals(expected));
 }