Function assertNever

app/src/lib/fatal-error.ts:18–20

(x: never, message: string)

 * @param message   The message to be used in the runtime exception.
 */
export function assertNever(x: never, message: string): never {
  throw new Error(message)
}

/**
 * Unwrap a value that, according to the type system, could be null or