text: /**
   * feature-check: set `_global_legacy_browser` variable according to the current browser's feature.
   * false if latest, 'legacy' if legacy, and 'hopeless' if not applicable of legacy builds.
   */
  (function(){
    _global_legacy_browser = false;
    try {
      // ES2017 async/await
      eval('(async function(){})');
      // ES2017 String.prototype.padStart
      ''.padStart();
      // ES2016 Array.includes.
      ['a', 'b', 'c'].includes('b');
      //
    } catch(e) {
      _global_legacy_browser = 'legacy';
      // Skip Googlebot.
      if (navigator.userAgent.indexOf("Googlebot")>=0) {
        return;
      }
      try {
        // Check basic DOM feature.
        document.documentElement.classList.contains;
        document.documentElement.dataset.foobar;
      } catch(e) {
        _global_legacy_browser = 'hopeless';
      }
      return;
    }
  })();
  