async function main() {
  try {
    // Get a list of all *_test.html files.
    const allTestHtmls = [
      ...await readdirRecursive(CLOSURE_PATH),
      ...await readdirRecursive(THIRD_PARTY_PATH),
    ].filter(f => f.endsWith('_test.html'));
    if (allTestHtmls.length === 0) {
      throw new Error(
          'No *_test.html files found. Did you run `npm run gen_test_htmls`?');
    }

    const output = [
      HEADER, '', 'var _allTests = [', ...allTestHtmls.map(f => `  '${f}',`),
      '];', '', FOOTER
    ];
    console.log(output.join('\n'));
    return 0;
  } catch (e) {
    console.error(e);
    return 1;
  }
}
Prints the generated alltests.js contents.
@return {!Promise<number>} The exit code.