null_safety rebuild script (#1369) (#1371)

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel channel on [Discord].

<!-- Links -->
[Flutter Style Guide]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]: https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

platform_channels/.metadata

@@ -1,10 +1,33 @@
 # This file tracks properties of this Flutter project.
 # Used by Flutter tool to assess capabilities and perform upgrades etc.
 #
-# This file should be version controlled and should not be manually edited.
+# This file should be version controlled.
 
 version:
-  revision: 5f21edf8b66e31a39133177319414395cc5b5f48
+  revision: f1875d570e39de09040c8f79aa13cc56baab8db1
   channel: stable
 
 project_type: app
+
+# Tracks metadata for the flutter migrate command
+migration:
+  platforms:
+    - platform: root
+      create_revision: f1875d570e39de09040c8f79aa13cc56baab8db1
+      base_revision: f1875d570e39de09040c8f79aa13cc56baab8db1
+    - platform: android
+      create_revision: f1875d570e39de09040c8f79aa13cc56baab8db1
+      base_revision: f1875d570e39de09040c8f79aa13cc56baab8db1
+    - platform: ios
+      create_revision: f1875d570e39de09040c8f79aa13cc56baab8db1
+      base_revision: f1875d570e39de09040c8f79aa13cc56baab8db1
+
+  # User provided section
+
+  # List of Local paths (relative to this file) that should be
+  # ignored by the migrate tool.
+  #
+  # Files that are not part of the templates will be ignored by default.
+  unmanaged_files:
+    - 'lib/main.dart'
+    - 'ios/Runner.xcodeproj/project.pbxproj'