[dartdev][flutter_tools] Rerunning commands needs to be a NOP for assets if hooks were cached

[dcharkes]

After the hooks are run in dartdev/flutter_tools, the assets might be copied/signed/install_name modified etc.

We'd like to ensure that we can cache those steps.

We cannot blindly rely on timestamps, as a hook might return a file with an older timestamp (on Windows downloaded files have their creation date as lastmodified, not the downloaded time). We could fix this by mandating that assets should have monotonically increasing timestamps, but that puts burden on the hook writer.

Maybe there are better solutions (using a hash, ...).

Also flutter run and flutter build --release copy files both to build/native_assets/* which also breaks the NOP-ness.

To investigate

• What does flutter_tools currently do? (Possibly content hashes https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/build_system/build_system.dart#L1174)

Context:

• [native_assets_builder] Set the last modified date of assets to the build output timestamp #1533 (comment)
• https://dart-review.googlesource.com/c/sdk/+/381580/5/pkg/dartdev/lib/src/native_assets_bundling.dart

This needs to play well with concurrent invocations as well:

• [native_assets_builder] Concurrency between Dart runs and asset edits #1534

Thanks @mkustermann and @blaugold