Make JSON.stringify call toJson() on objects that it can't serialize.

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('dart:json');
 #import('dart:html');
 #import('../../pkg/unittest/unittest.dart');
 #import('../../pkg/unittest/html_config.dart');
@@ skipping 35 matching lines @@
         equals({"x":3, "y": -4.5,  "z" : "hi", "u" : true, "v": false }));
 
     expect(JSON.parse(' {"x":3, "y": -4.5,  "z" : "hi" } '),
         equals({"x":3, "y": -4.5,  "z" : "hi" }));
 
     expect(JSON.parse(' {"y": -4.5,  "z" : "hi" ,"x":3 } '),
         equals({"y": -4.5,  "z" : "hi" ,"x":3 }));
 
     expect(JSON.parse('{ " hi bob " :3, "": 4.5}'),
         equals({ " hi bob " :3, "": 4.5}));
-  
+
     expect(JSON.parse(' { "x" : { } } '), equals({ 'x' : {}}));
     expect(JSON.parse('{"x":{}}'), equals({ 'x' : {}}));
 
     // Nulls are tricky.
     expect(JSON.parse('{"w":null}'), equals({ 'w' : null}));
 
     expect(JSON.parse('{"x":{"w":null}}'), equals({"x":{"w":null}}));
 
     expect(JSON.parse(' {"x":3, "y": -4.5,  "z" : "hi",'
                    '"w":null, "u" : true, "v": false } '),
@@ skipping 38 matching lines @@
 
     // Dart does not guarantee an order on the keys
     // of a map literal, so reparse and compare to the original Map.
     validateRoundTrip(
         {'x':3, 'y':-4.5, 'z':'hi', 'w':null, 'u':true, 'v':false});
     validateRoundTrip({"x":3, "y":-4.5, "z":'hi'});
     validateRoundTrip({' hi bob ':3, '':4.5});
     validateRoundTrip(
         {'x':{'a':3, 'b':-4.5}, 'y':[{}], 'z':'hi', 'w':{'c':null, 'd':true},
                   'v':null});
+
+    expect(JSON.stringify(new ToJson(4)), "4");
+    expect(JSON.stringify(new ToJson([4, "a"])), '[4,"a"]');
+    expect(JSON.stringify(new ToJson([4, new ToJson({"x":42})])),
+           '[4,{"x":42}]');
+
+    Expect.throws(() {
+      JSON.stringify([new ToJson(new ToJson(4))]);
+    });
+
+    Expect.throws(() {
+      JSON.stringify([new Object()]);
+    });
+
   });
 
   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()), throws);
     });
   });
 }
 
 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.parse(JSON.stringify(expected)), equals(expected));
 }