avoid duplicate ids in ng-repeated span (Chap. 6-7)

Oups, sorry, missed these before; this time, making the fix in all
remaining chapters (lib/component/search_recipe_component.html).
Drop cssUrl parameter to NgComponent since .css is empty (Chap. 6-7)

The component folder holds the files for the two components
search_recipe and view_recipe. Given that one of these has an empty
.css file, it seems reasonable to drop it, reducing the clutter and
driving home the fact that the `cssUrl` parameter is optional.
Merge pull request #52 from chalin/patch-6-drop-empty-component-css

Drop cssUrl parameter to NgComponent since .css is empty (Chap. 6-7)
Merge pull request #50 from chalin/patch-5-remove-id-from-repeated-span

avoid duplicate ids in ng-repeated span (Chap. 6-7)
fix ch07 use of shadow_dom package

See https://github.com/angular/angular.dart.tutorial/pull/28 for details.

Review URL: https://codereview.appspot.com/57990043