mara's first deploy hit:
Error: Couldn't resolve the package 'matrix' in 'package:matrix/matrix.dart'.
/nix/store/k9j8ns45fz7rpjp6rzk33ydjng67pgm0-source/web/native_executor.dart:1:8:
Error: Not found: 'package:matrix/matrix.dart'
Root cause: `dart compile js` walks up from the source file's dir to
find `.dart_tool/package_config.json`. My previous postInstall passed
`$src/web/native_executor.dart` — pointing dart at the unpacked nix
source, which has no `.dart_tool/` (pub-get wrote it to the build CWD,
not the read-only store path).
Fix: use a relative path `web/native_executor.dart`. nixpkgs's
buildFlutterApplication leaves CWD at the source root in postInstall
(its installPhase is just `cp -r build/web "$out"` with no `cd`
first — see `pkgs/development/compilers/flutter/build-support/
build-flutter-application.nix`), so the relative path walks up from
`web/` to the build CWD where pub-get's package_config lives.
Verified by `nix eval`; full closure build pending operator deploy.
Followup to #697 (the original fix; merged but mara's deploy then
surfaced this regression).
7647969d49