This is because the request actually completes successfully.
Your request to "login" will actually call http://127.0.0.1:6521/[Path_to_your_Dart_file]/login
The server started by Dart when running in Dartium (127.0.0.1:6521) seems to answer to every POST request with HTTP 200 and an empty response body.
If you change the method from POST to GET, it will fail as expected.
As for why the server does this - I don't really know. This would have to be answered by the Dart team.