Fix cycle

This assertion was only added to satisfy typescript. It creates a module cycle and isn't worth it.

Author: ef4

packages/@ember/engine/instance.ts

@@ -12,7 +12,7 @@ import { ContainerProxyMixin, RegistryProxyMixin } from '@ember/-internals/runti
 import type { InternalOwner } from '@ember/-internals/owner';
 import type Owner from '@ember/-internals/owner';
 import { type FullName, isFactory } from '@ember/-internals/owner';
-import Engine from '@ember/engine';
+import type Engine from '@ember/engine';
 import type Application from '@ember/application';
 import type { BootEnvironment } from '@ember/-internals/glimmer';
 import type { SimpleElement } from '@simple-dom/interface';
@@ -199,16 +199,14 @@ class EngineInstance extends EmberObject.extend(RegistryProxyMixin, ContainerPro
     @return {EngineInstance,Error}
   */
   buildChildEngineInstance(name: string, options: EngineInstanceOptions = {}): EngineInstance {
-    let ChildEngine = this.lookup(`engine:${name}`);
+    let ChildEngine = this.lookup(`engine:${name}`) as Engine;
 
     if (!ChildEngine) {
       throw new Error(
         `You attempted to mount the engine '${name}', but it is not registered with its parent.`
       );
     }
 
-    assert('expected an Engine', ChildEngine instanceof Engine);
-
     let engineInstance = ChildEngine.buildInstance(options);
 
     setEngineParent(engineInstance, this);