fix(upgrade): fix downgrade content projection and injector inheritance

- Full support for content projection in downgraded Angular 2
  components. In particular, this enables multi-slot projection and
  other features on <ng-content>.
- Correctly wire up hierarchical injectors for downgraded Angular 2
  components: downgraded components inherit the injector of the first
  other downgraded Angular 2 component they find up the DOM tree.

Closes #6629, #7727, #8729, #9643, #9649, #12675

modules/@angular/core/src/linker/compiler.ts

@@ -82,6 +82,14 @@ export class Compiler {
     throw _throwError();
   }
 
+  /**
+   * Exposes the CSS-style selectors that have been used in `ngContent` directives within
+   * the template of the given component.
+   * This is used by the `upgrade` library to compile the appropriate transclude content
+   * in the Angular 1 wrapper component.
+   */
+  getNgContentSelectors(component: Type<any>): string[] { throw _throwError(); }
+
   /**
    * Clears all caches.
    */