Split the initialization the _flutter namespace and the loader object.

Author: ditman

lib/web_ui/flutter_js/src/flutter.js

@@ -5,7 +5,9 @@
 import { FlutterLoader } from './loader.js';
 
 if (!window._flutter) {
-  window._flutter = {
-    loader: new FlutterLoader()
-  };
+  window._flutter = {};
+}
+
+if (!window._flutter.loader) {
+  window._flutter.loader = new FlutterLoader();
 }