add stub for web update scan window impl

@@ -61,6 +61,8 @@ class MobileScannerWebPlugin {
         return _torch(call.arguments);
         return _torch(call.arguments);
       case 'stop':
       case 'stop':
         return cancel();
         return cancel();
+      case 'updateScanWindow':
+        return Future<void>.value();
       default:
       default:
         throw PlatformException(
         throw PlatformException(
           code: 'Unimplemented',
           code: 'Unimplemented',