The macOS changes for the Hardware Keyboard project.

When replacing an int/float/bool attribute on the test node, we want to
erase the range from the iterator returned by remove_if to end(), which
may be an empty range. In the case where remove_if() returns end(), the
single-parameter variant of erase() attempts to remove an element past
the end of the container.

Some flutter_tester scripts rely on the old value because Dart will convert
"main.dart" into a Platform.script URI containing a fully qualified path to
the script.

* Switch build to using generated headers

* Update DEPS to pick up newer buildroot

* Only run hook on Windows

https://github.com/flutter/flutter/issues/77600

* Make window.locale(s) non-null

* upgrade deps to null-safe; migrate some test libs