After flutter/engine#41011 started publishing artifacts I tried to make a macOS Share extension, but on launch the engine will not start:

Failed to find path for "flutter_assets"
embedder.cc (1601): 'FlutterEngineInitialize' returned 'kInvalidArguments'. The assets path in the Flutter project arguments was missing.

The App.framework bundle isn't found from the appex bundle. Suspect macOS needs similar logic to this:

if ([mainBundle.bundleURL.pathExtension isEqualToString:@"appex"]) {

https://github.com/flutter/engine/pull/40332/files#diff-aff27bb72cc390eb03f0b733e1fa490102e311ffac10e8e09e8245e582484220R84

Here:
https://github.com/flutter/engine/blob/b50954c3f7d6dca6702ad843aa0fa7a51c774323/shell/platform/darwin/macos/framework/Source/FlutterDartProject.mm#L32

See also https://github.com/flutter/engine/pull/37421/files#diff-8646d867d6b502217ca15af1cfd042f7148929efb68e1f28d796351a80594832R38 which is changing this same spot.

Steps to reproduce

1. Create Flutter project

$ flutter create test_create
$ cd test_create
$ flutter build macos --config-only
$ open macos/Runner.xcworkspace/

2. Add a new Share target and Activate it.

3. In test_create/macos/FlutterShare/Info.plist change NSExtensionPrincipalClass to FlutterViewController.

4. In Xcode set a signing Team and set the Signing Certificate to Development for both the Runner and new FlutterShare targets. These app extensions fail with codesign errors if they are ad-hoc signed.

5. In the FlutterShare target build settings add -framework FlutterMacOS to Other Linker Flags

6. Run the FlutterShare target in Safari

7. In the top right corner of Safari under the Share menu choose FlutterShare

Results

In Xcode console:

2023-04-12 18:17:45.807583-0700 FlutterShare[28145:12591166] Failed to find path for "flutter_assets"
2023-04-12 18:17:45.808568-0700 FlutterShare[28145:12591166] Failed to find path for "flutter_assets"
embedder.cc (1601): 'FlutterEngineInitialize' returned 'kInvalidArguments'. The assets path in the Flutter project arguments was missing.
2023-04-12 18:17:45.808903-0700 FlutterShare[28145:12591166] Failed to initialize Flutter engine: error 2

and no UI launches.

cc @cyanglaz part of #124290