/** Mark a service or component instance for state preservation across
 *  full Angular HMR re-bootstraps. Call once from the constructor or
 *  `ngOnInit`. Safe in production (no-op outside dev mode).
 *
 *  @param instance Usually `this`. The class name is used as part of
 *      the cache key.
 *  @param key Optional discriminator when multiple instances of the
 *      same class can be alive at once (rows, tabs, etc). Coerced
 *      to string. Use `ngOnInit` to call this when the key depends
 *      on `@Input` values, since Angular sets inputs between
 *      constructor and ngOnInit. */
export declare const preserveAcrossHmr: (instance: object, key?: string | number) => void;