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/ios/.gitignore

@@ -1,3 +1,4 @@
+**/dgph
 *.mode1v3
 *.mode2v3
 *.moved-aside
@@ -18,10 +19,10 @@ Flutter/App.framework
 Flutter/Flutter.framework
 Flutter/Flutter.podspec
 Flutter/Generated.xcconfig
+Flutter/ephemeral/
 Flutter/app.flx
 Flutter/app.zip
 Flutter/flutter_assets/
-Flutter/.last_build_id
 Flutter/flutter_export_environment.sh
 ServiceDefinitions.json
 Runner/GeneratedPluginRegistrant.*