[flutter_tools] check if requireloader is defined (#55085)

packages/flutter_tools/lib/src/web/bootstrap.dart

@@ -62,9 +62,10 @@ define("main_module.bootstrap", ["$entrypoint", "dart_sdk"], function(app, dart_
   /* MAIN_EXTENSION_MARKER */
   child.main();
 
-window.\$dartLoader = {};
-window.\$dartLoader.rootDirectories = [];
-window.\$requireLoader.getModuleLibraries = dart_sdk.dart.getModuleLibraries;
+  window.\$dartLoader = {};
+  window.\$dartLoader.rootDirectories = [];
+  if (window.\$requireLoader) {
+    window.\$requireLoader.getModuleLibraries = dart_sdk.dart.getModuleLibraries;
     if (window.\$dartStackTraceUtility && !window.\$dartStackTraceUtility.ready) {
       window.\$dartStackTraceUtility.ready = true;
       let dart = dart_sdk.dart;
@@ -77,6 +78,7 @@ window.\$requireLoader.getModuleLibraries = dart_sdk.dart.getModuleLibraries;
         return dart.getSourceMap(url);
       });
     }
+  }
 });
 ''';
 }