lib/wda/utils.js
async function getWDAUpgradeTimestamp (bootstrapPath) {
  const carthageRootPath = path.resolve(bootstrapPath, CARTHAGE_ROOT);
  if (await fs.exists(carthageRootPath)) {
    const {mtime} = await fs.stat(carthageRootPath);
    return mtime.getTime();
  }
  return null;
}
async function getWDAUpgradeTimestamp (bootstrapPath) {
  const carthageRootPath = path.resolve(bootstrapPath, CARTHAGE_ROOT);
  if (await fs.exists(carthageRootPath)) {
    const {mtime} = await fs.stat(carthageRootPath);
    return mtime.getTime();
  }
  return null;
}
Retrieves WDA upgrade timestamp

@param {string} bootstrapPath The full path to the folder where WDA source is located
@return {?number} The UNIX timestamp of the carthage root folder, where dependencies are downloaded.
This folder is created only once on module upgrade/first install.