Handle exist sync

Author: ben-milanko

flutter_cache_manager/lib/src/cache_store.dart

@@ -1,6 +1,7 @@
 import 'dart:async';
 import 'dart:io';
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter_cache_manager/flutter_cache_manager.dart';
 
 import 'storage/file_system/file_system.dart';
@@ -187,7 +188,7 @@ class CacheStore {
     }
     final file = await fileSystem.createFile(cacheObject.relativePath);
 
-    if (file.existsSync()) {
+    if (kIsWeb ? await file.exists() : file.existsSync()) {
       try {
         await file.delete();
         // ignore: unused_catch_clause