code: function(win) {
      win = win || window;
      
      this.lastFocus = new Date().getTime();
      this._isFocused = true;
      
      var _self = this;
  
      // IE < 9 supports focusin and focusout events
      if ("onfocusin" in win.document) {
          event.addListener(win.document, "focusin", function(e) {
              _self._setFocused(true);
          });
  
          event.addListener(win.document, "focusout", function(e) {
              _self._setFocused(!!e.toElement);
          });
      }
      else {
          event.addListener(win, "blur", function(e) {
              _self._setFocused(false);
          });
  
          event.addListener(win, "focus", function(e) {
              _self._setFocused(true);
          });
      }
  }
docs: This class keeps track of the focus state of the given window.
  Focus changes for example when the user switches a browser tab,
  goes to the location bar or switches to another application.