Use onNodeCreated to set crossorigin: anonymous on requirejs scripts (#3031)

* Use onNodeCreated to set crossorigin: anonymous on requirejs scripts

* Add comment

Author: johnpryan

pkgs/dartpad_ui/lib/execution/frame.dart

@@ -88,11 +88,14 @@ window.onerror = function(message, url, line, column, error) {
 };
 ''');
 
+    // Set the crossorigin: anonymous attribute on require.js scripts.
+    // For example, dart_sdk.js or flutter_web.js.
     if (modulesBaseUrl != null) {
       script.writeln('''
 require.config({
   "baseUrl": "$modulesBaseUrl",
-  "waitSeconds": 60
+  "waitSeconds": 60,
+  "onNodeCreated": function(node, config, id, url) { node.setAttribute('crossorigin', 'anonymous'); }
 });
 ''');
     }