Dart request succeeding ... somehow?

Dart request succeeding ... somehow?

I'm developing a dart application which will consume a REST service I'm
building. I started writing out the dart code to perform an ajax request
to my login endpoint. However, even when my dart ajax request should fail,
it claims to succeed.
I don't have any services up and running (and even if I did it would be
using the wrong domain / port right now), but this code gives a 200 OK
HttpResponse every time:
class PlayerController {
const PlayerController();
static const String LOGIN_URL = "login";
void login(String username, String password) {
Map<String, String> headers = {"Content-Type":
"application/x-www-form-urlencoded"};
String body = "j_username=$username&j_password=$password&submit=Login";
HttpRequest.request(LOGIN_URL, method: "POST", requestHeaders:
headers, sendData: body)
.then((request) => processLogin(request, username))
.catchError((e) => processLoginError(e));
}
void processLogin(var whatIsThis, String username) {
query("#loginButton").text = "Logout";
//TODO get the player then set them
}
void processLoginError(var e) {
print("total failure to login because of $e");
}
}
It always hits the processLogin method, and never hits the
processLoginError method. Does anyone have any idea why this would be?
Should I be performing this ajax request in a different way? (If you
couldn't guess, it will be signing into spring security).
I read somewhere that file system requests always succeed. Is Dart somehow
making this a file system request rather than a web request?