message: docs(aio): http guide shows how to import toPromise operator from rxjs
  Solves
diff: @@ -407,6 +407,9 @@ like the Observable-based version.
  </div>
  +First, make sure to import the `toPromise` operator of the RxJS library.
  +
  +<code-example path="http/src/app/toh/hero.service.promise.ts" region="rxjs-imports" title="src/app/toh/hero.service.promise.ts (import rxjs)" linenums="false"></code-example>
  Here is a comparison of the `HeroService` using Promises versus Observables,
  highlighting just the parts that are different.
  