anchor.diff:  
     MDCMenu get _samplesMenu => _initSamplesMenu();
   
  -  Element? get _channelsDropdownButton =>
  -      querySelector('#channels-dropdown-button');
  +  ButtonElement get _channelsDropdownButton =>
  +      querySelector('#channels-dropdown-button') as ButtonElement;
   
     bool get _isCompletionActive => editor.completionActive;
   
  
positive.code:       return;
      }
      queryParams.channel = channel;
      dartServices.rootUrl = Channel.urlMapping[channel]!;
      updateVersions();
      performAnalysis();
  
negative.code:     _channelsMenu?.unlisten('MDCMenu:selected', _handleChannelsMenuSelected);
      _channelsMenu = MDCMenu(menuElement)
        ..setAnchorCorner(AnchorCorner.bottomLeft)
        ..setAnchorElement(_channelsDropdownButton)
        ..hoistMenuToBody();
  
      _channelsMenu?.listen('MDCMenu:selected', _handleChannelsMenuSelected);
  