Get rid of a couple unsafe casts

- use `assert`s in `setup-application-context` for router lookups
- bind an intermediate term in a conditional in `build-registry` to
  preserve the control-flow-based narrowing in the loop

addon-test-support/@ember/test-helpers/-internal/build-registry.ts

@@ -29,13 +29,12 @@ function exposeRegistryMethodsWithoutDeprecations(container: any) {
   ];
 
   for (let i = 0, l = methods.length; i < l; i++) {
-    let method = methods[i];
+    let methodName = methods[i];
 
-    if (method && method in container) {
-      container[method] = function (...args: unknown[]) {
-        // SAFETY: `method` is defined because we *just* checked that it is in
-        // the conditional wrapping this.
-        return container._registry[method!](...args);
+    if (methodName && methodName in container) {
+      const knownMethod = methodName;
+      container[knownMethod] = function (...args: unknown[]) {
+        return container._registry[knownMethod](...args);
       };
     }
   }

addon-test-support/@ember/test-helpers/setup-application-context.ts

@@ -12,6 +12,8 @@ import settled from './settled';
 import getTestMetadata from './test-metadata';
 import { runHooks } from './-internal/helper-hooks';
 import { Router } from '@ember/routing';
+import { RouterService } from '@ember/routing/router-service';
+import { assert } from '@ember/debug';
 
 export interface ApplicationTestContext extends TestContext {
   element?: Element | null;
@@ -88,17 +90,24 @@ export function setupRouterSettlednessTracking() {
   HAS_SETUP_ROUTER.set(context, true);
 
   let { owner } = context;
-  let router: Router;
+  let router: Router | RouterService;
   if (CAN_USE_ROUTER_EVENTS) {
-    router = owner.lookup('service:router') as Router;
+    let routerService = owner.lookup('service:router');
+    assert(
+      'router service is not available',
+      routerService instanceof RouterService
+    );
+    router = routerService;
 
     // track pending transitions via the public routeWillChange / routeDidChange APIs
     // routeWillChange can fire many times and is only useful to know when we have _started_
     // transitioning, we can then use routeDidChange to signal that the transition has settled
     router.on('routeWillChange', () => (routerTransitionsPending = true));
     router.on('routeDidChange', () => (routerTransitionsPending = false));
   } else {
-    router = owner.lookup('router:main') as Router;
+    let mainRouter = owner.lookup('router:main');
+    assert('router:main is not available', mainRouter instanceof Router);
+    router = mainRouter;
     ROUTER.set(context, router);
   }