I've seen a lot of players die to this very predictable and straightforward opening play from Aeon phantoms, so here is a quick 101 to countering it. First the relevant numbers:
Donut: 45000 mass, 33750 buildtime
40(forty!)x Mobile flak 6400 mass, 32000 buildtime
Those 40 flak will need 10 seconds to kill a full hp Czar and cost only a pittance compared to it, the problem is buildtime; when next to an Aeon neighbor you need to prepare for a potential rush before he starts the Czar, if he breaks and you dont have a single t2 landfactory it's too late. Luckily there is no real opportunity cost to building a few extra t2 support facs early (unless you're cybran) since hoverspam is not a terrible move on Roanoke.
Here's a good plan to follow: after storaging all t2 mexes make 6-8 t1 factories on your main island (to spam t1 engies), always quickly upgrade your main landfactory to t2 so if your Aeon neighbor looks menacing you can quickly pump out dozens of t2 flak with the assistance of your spammed engies (that otherwise would be upgrading your eco).