Function assertNonNullable

app/src/lib/fatal-error.ts:42–49

(
  x: T,
  message: string
)

 * value is defined.
 */
export function assertNonNullable<T>(
  x: T,
  message: string
): asserts x is NonNullable<T> {
  if (x == null) {
    return fatalError(message)
  }
}