Use case

flutter build windows --release -v

Shows extra 5 seconds spend somewhere. Loading the .sln file, using the "Build Logging"
extensions, shows 5 seconds spend everytime the app is build (Ctrl+Shift+B) from Visual Studio.

This led to the discovery it's due to the _phony_ target that makes it rebuilt everytime.

Now, while hot-reload is great and one of the best selling thing about flutter, a slow initial
build time, or slow in general build time if you work from the C++ side is not great.

Removing the _phony_ target from CMakeLists.txt made the problem go away
(from 11secs -> 6secs compile), but not sure why it was put in the first place, or
whether it's safe to do so.

Proposal

Look into whether it'll be possible to shed these 5 secs (50% of the time right now, spent
on my machine to flutter build windows --release .

with_phony.txt
without_phony.txt