I upgraded Dart to 2.8.4 and Aqueduct to 4.0.0-b1.
On the linux server I was starting the server with a daemon like this:
[Unit]
Description=My server
[Service]
User=web
WorkingDirectory=/home/web/dart/my_server
ExecStart=/home/web/.pub-cache/bin/aqueduct serve
Restart=always
[Install]
WantedBy=multi-user.target
But Aqueduct won't start now.