requestCb = function() {
    if (request.readyState == 4) {
      if (request.status == 200) {
        var success = Ardublockly.replaceBlocksfromXml(request.responseText);
        cbSuccess(success);
      } else {
        cbError();
      }
    }
  }
Loads an XML file from the server and replaces the current blocks into the
Blockly workspace.
@param {!string} xmlFile XML file path in a reachable server (no local path).
@param {!function} cbSuccess Function to be called once the file is loaded.
@param {!function} cbError Function to be called if there is a connection
    error to the XML server.