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

tools/public_api_guard/core/index.d.ts

@@ -219,6 +219,7 @@ export declare class Compiler {
     compileModuleAndAllComponentsSync<T>(moduleType: Type<T>): ModuleWithComponentFactories<T>;
     compileModuleAsync<T>(moduleType: Type<T>): Promise<NgModuleFactory<T>>;
     compileModuleSync<T>(moduleType: Type<T>): NgModuleFactory<T>;
+    getNgContentSelectors(component: Type<any>): string[];
 }
 
 /** @experimental */