anchor.diff:  
     void showGist(RouteEnterEvent event) {
       String gistId = event.parameters['gist'];
  +    _clearOutput();
   
       if (!isLegalGistId(gistId)) {
         showHome(event);
  
positive.code:     runButton.disabled = true;
      overlay.visible = true;
  
      _clearOutput();
  
      Stopwatch compilationTimer = new Stopwatch()..start();
  
  
negative.code:     _clearOutput();
    }
  
    Future showHome(RouteEnterEvent event) async {
      // Don't auto-run if we're re-loading some unsaved edits; the gist might
      // have halting issues (#384).
      bool loadedFromSaved = false;
  