function getFiles(folder) {
  // Only gets Docs files.
  const files = folder.getFilesByType(MimeType.GOOGLE_DOCS);
  let docIDs = [];
  while (files.hasNext()) {
    let file = files.next();
    docIDs.push(file.getId());
  }
  return docIDs;
}
Gets the IDs of the Docs files within the folder that contains source files.

Called from function performImport().