function loadSampleDatafromJson(filename) {
  return new goog.Promise(function(resolve, reject) {
    goog.net.XhrIo.send(filename, function(e) {
      var xhr = e.target;
      resolve(JSON.parse(xhr.getResponseText()));
    });
  });
}
Reads the sample data from a JSON file.
@param {string} filename The name of the JSON file holding the data. Has to
    reside in the same folder as this test.
@return {!IThenable}