Correctly associate props with factory and owner in FactoryManager

Previously, we stomped the `props` binding with an `Object.assign()`,
which meant that the original empty props object would get GC'd after
the end of the method and the item passed into the class created at the
end of the `FactoryManager.create` call would be a *different* object,
which does *not* have the factory or owner associations.

Fixes #20023

packages/@ember/-internals/container/lib/container.ts

@@ -504,14 +504,10 @@ export class FactoryManager<T, C extends FactoryClass | object = FactoryClass> {
       );
     }
 
-    let props = {};
+    let props = options ? { ...options } : {};
     setOwner(props, container.owner!);
     setFactoryFor(props, this);
 
-    if (options !== undefined) {
-      props = Object.assign({}, props, options);
-    }
-
     if (DEBUG) {
       let lazyInjections;
       let validationCache = this.container.validationCache;

packages/@ember/-internals/routing/lib/services/routing.ts

@@ -12,7 +12,7 @@ import { Route } from '../..';
 import EmberRouter from '../system/router';
 import RouterState from '../system/router_state';
 
-const ROUTER = (symbol('ROUTER') as unknown) as string;
+const ROUTER = symbol('ROUTER');
 
 /**
   The Routing service is used by LinkTo, and provides facilities for

packages/@ember/-internals/routing/lib/system/router.ts

@@ -341,7 +341,7 @@ class EmberRouter<R extends Route = Route> extends EmberObject.extend(Evented) i
 
         assert('Route is unexpectedly missing an owner', routeOwner);
 
-        let route = routeOwner.lookup(fullRouteName) as R;
+        let route = routeOwner.lookup(fullRouteName) as R | undefined;
 
         if (seen[name]) {
           assert('seen routes should exist', route);
@@ -351,6 +351,8 @@ class EmberRouter<R extends Route = Route> extends EmberObject.extend(Evented) i
         seen[name] = true;
 
         if (!route) {
+          // SAFETY: this is configured in `commonSetupRegistry` in the
+          // `@ember/application/lib` package.
           let DefaultRoute: any = routeOwner.factoryFor('route:basic')!.class;
           routeOwner.register(fullRouteName, DefaultRoute.extend());
           route = routeOwner.lookup(fullRouteName) as R;
@@ -362,15 +364,15 @@ class EmberRouter<R extends Route = Route> extends EmberObject.extend(Evented) i
           }
         }
 
-        route!._setRouteName(routeName);
+        route._setRouteName(routeName);
 
-        if (engineInfo && !hasDefaultSerialize(route!)) {
+        if (engineInfo && !hasDefaultSerialize(route)) {
           throw new Error(
             'Defining a custom serialize method on an Engine route is not supported.'
           );
         }
 
-        return route!;
+        return route;
       }
 
       getSerializer(name: string) {