Update flutter from 2.2.0 to 2.2.2 (#5908)

packages/flutter.rb

@@ -3,7 +3,7 @@ require 'package'
 class Flutter < Package
   description "Flutter is Google's UI toolkit for building beautiful, natively compiled applications for mobile, web, and desktop from a single codebase."
   homepage 'https://flutter.dev/'
-  version '2.2.0'
+  version '2.2.2'
   license 'BSD-3'
   compatibility 'all'
   source_url 'https://github.com/flutter/flutter.git'