fix analysis issue

Author: renefloor

flutter_cache_manager/lib/src/cache_managers/image_cache_manager.dart

@@ -2,6 +2,7 @@ import 'dart:async';
 import 'dart:math';
 import 'dart:ui' as ui;
 
+import 'package:file/file.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_cache_manager/flutter_cache_manager.dart';