[cross_file] Correct sorting of import starting with dot-slash (flutter#4449)

Move an import directive to be in correct order. Fixes flutter#130434

Author: srawlins

packages/cross_file/lib/src/types/html.dart

@@ -9,8 +9,8 @@ import 'dart:typed_data';
 
 import 'package:meta/meta.dart';
 
-import './base.dart';
 import '../web_helpers/web_helpers.dart';
+import 'base.dart';
 
 // Four Gigabytes, in bytes.
 const int _fourGigabytes = 4 * 1024 * 1024 * 1024;