[native_assets_builder] Concurrent execution of NativeAssetsBuilder

[dcharkes]

Currently, NativeAssetsBuilders methods can be called by the embedder as follows:

• Concurrently running methods that work on different BuildConfig/LinkConfigs is fine. Everything is happening in the .dart_tool/native_assets_builder/<hashed_config>/.
• Concurrently running methods on the same config is not supported.

This is currently mostly fine.

The situation in which this breaks in Flutter is when flutter run -d all has overlapping devices. E.g. if there are two Android phones connected. (It works fine with MacOS, iOS Simuatlor, iOS device, and Android emulator together.)

Flutter does not allow running more than one flutter command in the terminal concurrently. It has some kind of lock.

Dart doesn't have any commands running multiple native asset builds.

However, Dart doesn't guard against concurrent invocations. So two concurrent dart run process runs will cause undefined behavior.

Also there's no guard against running dart and flutter concurrently.

We should explore making the NativeAssetsBuilder re-entrant. It should support the following scenarios:

• multiple invocations in the same Dart process Not needed.
• multiple invocations in different Dart processes
• it should keep working if any of these processes is abruptly terminated

Since multiple processes need to communicate, we'd likely want to use lock files on the file system.

dart:io surfaces locks: https://api.dart.dev/stable/3.4.4/dart-io/RandomAccessFile/lock.html

[dcharkes]

Due to flutter run -d all running native assets builds concurrently, we cannot share the hook compilation to kernel across the same hook but different config invocations in #1256.