Plausible, but i wouldn’t put odds on anything.
LUCKILY, for me because i really like Flutter, I think it hits critical mass soon and wouldn’t matter if Google killed Fuchsia.
From day 0 on launch, you can expect Fuchsia to run your Dart apps and to run existing Android apps via a tool called Machina (Zircon hypervisor running Linux kernel). If Fuchsia benefits from the Flutter apps being made and gives Google more complete control over the OS unlike Android, then it looks highly unlikely that it will be killed.
In reality as Fuchsia is being ’dog-fooded’ on Google devices it is increasingly likely that Android will be killed.
If Fuchsia ever gets released, I bet ART will replace Flutter.
It is already being ported to run on Fuchsia anyway.
Flutter has zero relationship with ART, it just plugs into Android via the NDK, like anyone else using C++ code.