anchor.diff:  Never TODO([String message = '']) => throw UnimplementedError(message);
   ''';
   
  -  String _decorateJavaScript(String javaScript, {String modulesBaseUrl}) {
  -    final postMessagePrint = '''
  +  String _decorateJavaScript(String javaScript,
  +      {@required String modulesBaseUrl}) {
  +    final completeScript = StringBuffer();
  +    // postMessagePrint:
  +    completeScript.writeln('''
   var testKey = '$testKey';
   
   function dartPrint(message) {
  
positive.code: }
  // Unload previous version.
  require.undef('dartpad_main');
  ''';
  
      /// The javascript exception handling for Dartpad catches both errors
      /// directly raised by main() (in which case we might have useful Dart
      /// exception information we don't want to discard), as well as errors
      /// generated by other means, like assertion errors when starting up
      /// asynchronous functions.
      ///
      /// To avoid duplicating error messages on the DartPad console, we signal to
      /// window.onerror that we've already sent a dartMainRunner message by
      /// flipping _thrownDartMainRunner to true.  Some platforms don't populate
      /// error so avoid using it if it is null.
      ///
      /// This seems to produce both the stack traces we expect in inspector
      /// and the right error messages on the console.
      const exceptionHandler = '''
  var _thrownDartMainRunner = false;
  
  window.onerror = function(message, url, lineNumber, colno, error) {
  
negative.code: 
  import 'dart:async';
  import 'dart:html';
  
  import 'package:meta/meta.dart';
  
  import 'execution.dart';
  
  