onError is not invoked when http.get fails

[traceypowersg]

_http.get('/i/do/not/exist')
  .then((HttpResponse response) {
    // don't expect this to be invoked. URL provided is a 404
  },
  onError: (Object obj) {
    print('heavens to betsy! that url was no good!');
  });

The log looks like this:
Failed to load resource: the server responded with a status of 404 (Not Found)
http://127.0.0.1:3030/i/do/not/exist
Instance of '_XMLHttpRequestProgressEvent@0x3918afae'

STACKTRACE:
null

But the error handler message never prints out in the console, nor does a breakpoint set there get hit.

[vsavkin]

@jbdeboer, this issue is caused by a bug in HttpBackend. I was going to submit a pull request with a fix. The fix itself is straightforward, but there are no tests for HttpBackend. And I am not sure if I should add one (since HttpBackend is just thin wrapper around dom.HttpRequest, so writing an integration test for it may be an overkill).