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/src/jit/compiler.ts

@@ -70,6 +70,14 @@ export class JitCompiler implements Compiler {
     return this._compileModuleAndAllComponents(moduleType, false).asyncResult;
   }
 
+  getNgContentSelectors(component: Type<any>): string[] {
+    const template = this._compiledTemplateCache.get(component);
+    if (!template) {
+      throw new Error(`The component ${stringify(component)} is not yet compiled!`);
+    }
+    return template.compMeta.template.ngContentSelectors;
+  }
+
   private _compileModuleAndComponents<T>(moduleType: Type<T>, isSync: boolean):
       SyncAsyncResult<NgModuleFactory<T>> {
     const loadingPromise = this._loadModules(moduleType, isSync);
@@ -408,6 +416,11 @@ class ModuleBoundCompiler implements Compiler {
     return this._delegate.compileModuleAndAllComponentsAsync(moduleType);
   }
 
+  getNgContentSelectors(component: Type<any>): string[] {
+    return this._delegate.getNgContentSelectors(component);
+  }
+
+
   /**
    * Clears all caches
    */

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(