降级platform_info库，以支持最小sdk版本3.0

Author: zhangzqs

base/lib/src/storage/config/host.dart

@@ -215,7 +215,7 @@ class DefaultHostProviderV2 extends HostFreezer {
       () async => BucketRegionsQuery.create(
         bucketHosts: _bucketHosts,
         useHttps: _useHttps,
-        persistentFilePath: platform.js // web端不要持久化到磁盘，内部自动使用内存缓存即可
+        persistentFilePath: platform.isWeb // web端不要持久化到磁盘，内部自动使用内存缓存即可
             ? null
             : join(
                 Directory.systemTemp.path,

base/lib/src/storage/task/request_task.dart

@@ -9,8 +9,8 @@ part 'request_task_controller.dart';
 part 'request_task_manager.dart';
 
 String _getDefaultUserAgent() {
-  return platform.js
-      ? 'QiniuDart/v$currentVersion (Web)'
+  return platform.isWeb
+      ? 'QiniuDart/v$currentVersion (Web;)'
       : 'QiniuDart/v$currentVersion (${SysInfo.kernelName} ${SysInfo.kernelVersion} ${SysInfo.kernelArchitecture}; ${SysInfo.operatingSystemName} ${SysInfo.operatingSystemVersion};)';
 }
 

base/pubspec.yaml

@@ -19,7 +19,7 @@ dependencies:
   convert: ^3.1.1
   cache_provider: ^1.0.1
   singleflight: ^1.0.2
-  platform_info: ^5.0.0
+  platform_info: ^4.0.0
 
 dev_dependencies:
   test: any