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/compiler/testing/index.ts

@@ -70,6 +70,10 @@ export class TestingCompilerImpl implements TestingCompiler {
     return this._compiler.compileModuleAndAllComponentsAsync(moduleType);
   }
 
+  getNgContentSelectors(component: Type<any>): string[] {
+    return this._compiler.getNgContentSelectors(component);
+  }
+
   overrideModule(ngModule: Type<any>, override: MetadataOverride<NgModule>): void {
     const oldMetadata = this._moduleResolver.resolve(ngModule, false);
     this._moduleResolver.setNgModule(