async function* yieldClosureFiles() {
  const {stdout} = await execP('git ls-files');
  const files = stdout.split('\n').filter(x => x);
  for (const filePath of files) {
    const ext = path.extname(filePath);
    if (IGNORED_EXTENSIONS.includes(ext) || IGNORED_FILES.includes(filePath) ||
        filePath.startsWith('third_party') || filePath.endsWith('/BUILD'))
      continue;
    yield {
      filePath,
      contents: (await fs.readFile(filePath, 'utf8')).split('\n')
    };
  }
}
Yield open-sourced Closure file paths and contents, one at a time.