feat(compiler): Added "strictMetadataEmit" option to ngc (#10951)

ngc can now validate metadata before emitting to verify it doesn't
contain an error symbol that will result in a runtime error if
it is used by the StaticReflector.

To enable this add the section,

  "angularCompilerOptions": {
    "strictMetadataEmit": true
  }

to the top level of the tsconfig.json file passed to ngc.

Enabled metadata validation for packages that are intended to be
used statically.

modules/@angular/router/testing/router_testing_module.ts

@@ -39,7 +39,12 @@ export class SpyNgModuleFactoryLoader implements NgModuleFactoryLoader {
   }
 }
 
-function setupTestingRouter(
+/**
+ * Router setup factory function used for testing.
+ *
+ * @experimental
+ */
+export function setupTestingRouter(
     urlSerializer: UrlSerializer, outletMap: RouterOutletMap, location: Location,
     loader: NgModuleFactoryLoader, compiler: Compiler, injector: Injector, routes: Route[][]) {
   return new Router(