anchor.diff:      // Set up iframe.
       executionService = new ExecutionService(_frame);
       executionService.onStdout.listen(_showOuput);
  -    executionService.onStderr.listen(_showErrorOuput);
  +    executionService.onStderr.listen((m) => _showOuput(m, error: true));
   
       runbutton = new DButton(querySelector('#runbutton'));
       runbutton.onClick.listen((e) {
  
positive.code:     _context.focus();
    }
  
    void _showOuput(String message) {
      _outputpanel.text += message;
    }
  
    void _showErrorOuput(String message) {
      SpanElement span = new SpanElement();
      span.classes.add('errorOutput');
      span.text = message;
      _outputpanel.children.add(span);
    }
  
    void _clearOutput() {
      _outputpanel.text = '';
    }
  }
  
  
negative.code:     }).catchError((e) {
        // TODO: Also display using a toast.
        _clearOutput();
        _showOuput('There was an issue when compiling to JavaScript:\n${e}',
            error: true);
      }).whenComplete(() {
        _context.markCssClean();
        runbutton.disabled = false;
  