[flutter] Fix .wasm lookup path on web.

spine-flutter/lib/init_web.dart

@@ -63,7 +63,7 @@ Future<SpineFlutterFFI> initSpineFlutterFFI() async {
     registerOpaqueType<spine_skin_entries_wrapper>();
 
     await js.importLibrary('$_basePath/libspine_flutter.js');
-    String path = 'libspine_flutter.wasm';
+    String path = '../packages/spine_flutter/assets/libspine_flutter.wasm';
     Uint8List wasmBinaries = (await rootBundle.load(path)).buffer.asUint8List();
     _module = await EmscriptenModule.compile(wasmBinaries, 'libspine_flutter');
   }

spine-flutter/pubspec.yaml

@@ -53,9 +53,9 @@ flutter:
         ffiPlugin: true
 
   # To add assets to your plugin package, add an assets section, like this:
-  # assets:
-  #   - images/a_dot_burr.jpeg
-  #   - images/a_dot_ham.jpeg
+  assets:
+    - lib/assets/libspine_flutter.js
+    - lib/assets/libspine_flutter.wasm
   #
   # For details regarding assets in packages, see
   # https://flutter.dev/assets-and-images/#from-packages