anchor.diff:      /// and the right error messages on the console.
       final String exceptionHandler = '''
   var _thrownDartMainRunner = false;
  -function dartMainRunner(main, args) {
  -  try {
  -    main(args);
  -  } catch(error) {
  -    parent.postMessage(
  -      {'sender': 'frame', 'type': 'stderr', 'message': "Uncaught exception:\\n" + error.message}, '*');
  -    _thrownDartMainRunner = true;
  -    throw error;
  -  }
  -}
   
   window.onerror = function(message, url, lineNumber, colno, error) {
     if (!_thrownDartMainRunner) {
  
positive.code:     String postfix = '';
      if (usesRequireJs) {
        postfix = '''
  require(["dartpad_main", "dart_sdk"], function(dartpad_main, dart_sdk) {
      // SDK initialization.
      dart_sdk.dart.setStartAsyncSynchronously(true);
  
negative.code:     String javaScript, {
      String modulesBaseUrl,
    }) {
      return _send('execute', {
        'html': html,
        'css': css,
        'js': _decorateJavaScript(javaScript, modulesBaseUrl: modulesBaseUrl),
      });
    }
  
  