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 _decorateJavaScript(String javaScript, {String modulesBaseUrl}) {
      final String postMessagePrint = '''
  const testKey = '$testKey';
  
  function dartPrint(message) {
    if (message.startsWith(testKey)) {
  
negative.code:     String postfix = '';
      if (usesRequireJs) {
        postfix = '''
  require(['dart_sdk'],
    function(sdk) {
      'use strict';
      sdk.developer._extensions.clear();
      sdk.dart.hotRestart();
  });
  
  require(["dartpad_main", "dart_sdk"], function(dartpad_main, dart_sdk) {
      // SDK initialization.
      dart_sdk.dart.setStartAsyncSynchronously(true);
  