anchor.diff:      }).catchError((e) {
         // TODO: Also display using a toast.
         _clearOutput();
  -      _showErrorOuput('Error compiling:\n${e}');
  +      _showOuput('There was an issue when compiling to JavaScript:\n${e}',
  +          error: true);
       }).whenComplete(() {
         _context.markCssClean();
         runbutton.disabled = false;
  
positive.code: 
    ExecutionService(this.frame) {
      window.onMessage.listen((MessageEvent event) {
        String message = '${event.data}\n';
  
        if (message.startsWith('stderr: ')) {
          _stderrController.add(message.substring(8));
  
negative.code:     // Set up iframe.
      executionService = new ExecutionService(_frame);
      executionService.onStdout.listen(_showOuput);
      executionService.onStderr.listen((m) => _showOuput(m, error: true));
  
      runbutton = new DButton(querySelector('#runbutton'));
      runbutton.onClick.listen((e) {
  