Fix JSON encoder accepting NaN and Infinity.

pkg/json/lib/json.dart

 // Copyright (c) 2012, 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.
 
 /**
  * Utilities for encoding and decoding JSON (JavaScript Object Notation) data.
  */
 
 library json;
 
@@ skipping 751 matching lines @@
   }
 
   /**
    * Serializes a [num], [String], [bool], [Null], [List] or [Map] value.
    *
    * Returns true if the value is one of these types, and false if not.
    * If a value is both a [List] and a [Map], it's serialized as a [List].
    */
   bool stringifyJsonValue(final object) {
     if (object is num) {
-      // TODO: use writeOn.
+      if (!object.isFinite) return false;
       sink.write(numberToString(object));
       return true;
     } else if (identical(object, true)) {
       sink.write('true');
       return true;
     } else if (identical(object, false)) {
       sink.write('false');
        return true;
     } else if (object == null) {
       sink.write('null');
@@ skipping 35 matching lines @@
         first = false;
       });
       sink.write('}');
       seen.remove(object);
       return true;
     } else {
       return false;
     }
   }
 }