toString issue

[freewind]

class User extends JsonObject {
    String name;
}

var user = new User();
user.name = "Mike";
print(user.toString());

Will print null.

I think it should always use JSON.stringify instead of _jsonString.

[chrisbu]

I've changed the toString functionality to use JSON.stringify, and added a unit test, but the code above is slightly wrong.
The name property on the User class is a "real" property, and won't be intercepted by JsonObject's noSuchMethod call, which, which is why it prints null.