anchor.diff:    We *can* add it if we really want to. It isn't necessary because the
     `HeroesComponent` is already marked with `@Component`, and this
     !{_decorator} class (like `@Directive` and `@Pipe`, which we'll learn about later)
  -  is a subtype of <a href="#{injMetaUrl}">InjectableMetadata</a>.  It is in
  -  fact `InjectableMetadata` #{_decorator}s that
  +  is a subtype of <a href="#{injUrl}">Injectable</a>.  It is in
  +  fact `Injectable` #{_decorator}s that
     identify a class as a target for instantiation by an injector.
   
   +ifDocsFor('ts')
  
positive.code:       for _every class with at least one decorator_.
  
        While any decorator will trigger this effect, mark the service class with the
        <a href="#{injMetaUrl}">InjectableMetadata</a> #{_decorator}
        to make the intent clear.
  
  .callout.is-critical
  
negative.code:   We call that property within our `getHeroes` method when anyone asks for heroes.
  
  //- FIXME refer to Dart API when that page becomes available.
  - var injUrl = 'https://angular.io/docs/ts/latest/api/core/index/Injectable-decorator.html';
  h3#injectable Why @Injectable()?
  :marked
    **<a href="#{injUrl}">@Injectable()</a>** marks a class as available to an
    injector for instantiation. Generally speaking, an injector will report an
    error when trying to instantiate a class that is not marked as
    `@Injectable()`.
  