HttpRequest load fix

[fsw]

Because this event was missing, using HttpRequest with onLoad listener was not working as listener was never called:

    var xhr = HttpRequest();
    xhr.open('GET', 'https://swift.shop', async: true);
    xhr.onLoad.listen((e) {
      print('LOADED');
    });
    xhr.send();

This event was also used internally in request method so methods like getString were not working as well:

    HttpRequest.getString('https://github.com').then((content) {
      print(content);
    });

I think that this is a correct moment to send this event according to specs. https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/load_event

I have added a test for getString method that fails without this fix.

[terrier989]

Thank you for the pull request again! 👍