terminalErrors constant
The error codes a client can never recover from by trying again.
A refusal of the build: only a newer one changes the answer, so a client that hears one stops instead of reconnecting. Every other code in this class names something that can pass. A new terminal code belongs here, and nowhere else — the clients read this set rather than each naming the codes themselves.
Implementation
static const Set<String> terminalErrors = {
errorUnsupportedProtocolVersion,
errorUnsupportedClient,
};