In JsonEncoder, test if map has only string keys, and use toEncodable if not.

tests/lib/convert/json_toEncodable_reviver_test.dart

 // Copyright (c) 2014, 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:expect/expect.dart';
 import 'dart:convert';
 
 class A {
   final x;
@@ skipping 20 matching lines @@
   Expect.isTrue(decoded[0] is A);
   Expect.equals(0, decoded[0].x);
   Expect.isTrue(decoded[1] is Map);
   Expect.isNotNull(decoded[1]["2"]);
   Expect.isTrue(decoded[1]["2"] is A);
   Expect.equals(1, decoded[1]["2"].x);
 
   var a = extendedJson.decode(extendedJson.encode(new A(499)));
   Expect.isTrue(a is A);
   Expect.equals(499, a.x);
+
+  testInvalidMap();
 }
+
+
+void testInvalidMap() {
+  var map = {"a" : 42, "b": 42, 37: 42};  // Non-string key.
+  var enc = new JsonEncoder((_) => "fixed");
+  var res = enc.convert(map);
+  Expect.equals('"fixed"', res);
+
+  enc = new JsonEncoder.withIndent(" ", (_) => "fixed");
+  res = enc.convert(map);
+  Expect.equals('"fixed"', res);
+}