Force all dart:io usage to go through 'base/io.dart' (#7390)

This ensures that accidental usages of dart:io's
file API don't creep in over time.

packages/flutter_tools/lib/src/version.dart

diff --git a/packages/flutter_tools/lib/src/version.dart b/packages/flutter_tools/lib/src/version.dart
index 953d41e..579290d 100644
--- a/packages/flutter_tools/lib/src/version.dart
+++ b/packages/flutter_tools/lib/src/version.dart
@@ -2,8 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:io';
-
+import 'base/io.dart';
 import 'base/process.dart';
 import 'base/process_manager.dart';
 import 'cache.dart';