async function getColumnNamesHandler() {
  ui.updateColumnNamesOutput([]);
  const url = ui.getQueryElement().value;
  ui.updateStatus(`Attempting to connect to CSV resource at ${url}`);
  const myData = tf.data.csv(url);
  ui.updateStatus('Got the data connection ... determining the column names');
  ui.updateColumnNamesMessage('Determining column names.');
  try {
    const columnNames = await myData.columnNames();
    ui.updateStatus('Done getting column names.');
    ui.updateColumnNamesMessage('');
    ui.updateColumnNamesOutput(columnNames);
  } catch (e) {
    const errorMsg = `Caught an error retrieving column names from ${url}.  ` +
        `This URL might not be valid or might not support CORS requests.` +
        `  Check the developer console for CORS errors.` + e;
    ui.updateColumnNamesMessage(errorMsg);
    return;
  }
}
Builds a CSV Dataset object using the URL specified in the UI.  Then connects
with the dataset object to retrieve the column names.  Updates the UI
accordingly.