Add scenario_app target to iOS scenario test README

[jmagman]

Just building the ios_debug_sim_unopt does not generate the scenario_app. Add instructions to the README to also build the scenario_app target.

See #27302

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[jmagman]

Just building ios_debug_sim_unopt ios_test_flutter didn't generate out/ios_debug_sim_unopt/scenario_app.
@dnfield I don't see where this is done on the recipe side, am I missing something here?

https://flutter-review.googlesource.com/c/recipes/+/15820

[dnfield]

It should have - when I test a local build it's working fine, and it should get pulled in by default from the root BUILD.gn here:

engine/BUILD.gn

Lines 133 to 136 in 5f4c237

	if ((flutter_runtime_mode == "debug" || flutter_runtime_mode == "profile") &&
	(is_ios || is_android)) {
	public_deps += [ "//flutter/testing/scenario_app" ]
	}

[dnfield]

explicitly building that target may not be a bad idea because in theory it'll be faster by excluding other targets you may not need, but in practice I doubt it's much different.

So these instructions should not be necessary but they're not wrong either...

[jmagman]

I see what happened, locally I was running the target ios_test_flutter, which doesn't add the scenario_app dep. Just building ios_debug_sim_unopt on its own (as the README says now) did work.

$ ninja -C out/ios_debug_sim_unopt ios_test_flutter
$ ll out/ios_debug_sim_unopt/scenario_app
ls: out/ios_debug_sim_unopt/scenario_app: No such file or directory
$ ninja -C out/ios_debug_sim_unopt
$ ll out/ios_debug_sim_unopt/scenario_app
total 24
...

Thanks for the sanity check @dnfield.