[native_assets_builder] Concurrency between Dart runs and asset edits

[dcharkes]

If we have a long running Dart program that's running in the background and using assets, and at the same time we try to modify asset files (either by hand, or by rerunning a hook), we run into file locking issues on Windows.

• Modifying a asset/my_data.txt while a Dart program has it already open.
• Modifying a C program while the Dart program is already running, and then running the same program in another terminal concurrently, triggering the hook again.

We either need to:

1. Ensure we copy files out, so that after the native_assets_builder is done running, Dart/Flutter only use copies of the files produced by hooks. Originally posted by @dcharkes in [native_assets_builder] Set the last modified date of assets to the build output timestamp #1533 (comment)
2. Or, we should provide some reasonable error messages. But how do we do that? Locking the asset build directories and asset files while running?

Thanks @blaugold for bringing this up!