Make JSON encoder take extra function argument to use instead of toJson calls.

tests/json/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:expect/expect.dart";
 import "dart:convert";
 import 'dart:html';
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
@@ skipping 118 matching lines @@
 
   });
 
   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.encode(new TestClass()), throws);
   });
+
+  test('stringify custom converter', () {
+    List l = [const C(0), const C(1)];
+    expect(JSON.encode(l, (x) => "C(${x.id})"), equals("[C(0),C(1)]"));
+    expect(JSON.encode(l), equals("[C/0,C/1]"));
+  });
+}
+
+class C {
+  final int id;
+  const C(this.id);
+  toJson() => "C/$id";
 }
 
 class TestClass {
   int x;
   String y;
 
   TestClass() : x = 3, y = 'joe' { }
 }
 
 class ToJson {
   final object;
   const ToJson(this.object);
   toJson() => object;
 }
 
 /**
  * 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));
 }
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