message: asserting overriden props in AddressableCard are readonly (probably not necessary--but just in case)
diff: @@ -902,8 +902,8 @@ export class FieldCard extends Card {
  export class AddressableCard extends Card implements CardId {
  // these are non-null because of the assertion in our construction that
  // ensures csId is present.
  - csId!: string;
  - upstreamId!: NonNullable<Card['upstreamId']>;
  + readonly csId!: string;
  + readonly upstreamId!: NonNullable<Card['upstreamId']>;
  constructor(jsonapi: SingleResourceDoc, service: ScopedCardService, identity?: CardId) {
  let actualRealm = identity?.csRealm ?? jsonapi.data.attributes?.csRealm;
  