Refactor Ember.ApplicationInstance destruction.

Update `EmberApp.prototype.visitRoute` to ensure that the
`Ember.ApplicationInstance` instance that we create is _always_
`.destroy()`ed (using `try`/`finally`).

src/ember-app.js

@@ -253,13 +253,20 @@ class EmberApp {
    */
   async visitRoute(path, fastbootInfo, bootOptions, result) {
     let instance = await this.buildAppInstance();
-
     result.instance = instance;
-    registerFastBootInfo(fastbootInfo, instance);
 
-    await instance.boot(bootOptions);
-    await instance.visit(path, bootOptions);
-    await fastbootInfo.deferredPromise;
+    try {
+      registerFastBootInfo(fastbootInfo, instance);
+
+      await instance.boot(bootOptions);
+      await instance.visit(path, bootOptions);
+      await fastbootInfo.deferredPromise;
+    } finally {
+      // ensure we invoke `instance.destroy()`, but use
+      // `result._destroyAppInstance()` so that the `result` object's internal
+      // `this._instanceDestroyed` flag is correct
+      result._destroyAppInstance();
+    }
   }
 
   /**
@@ -328,11 +335,7 @@ class EmberApp {
       // eslint-disable-next-line require-atomic-updates
       result.error = error;
     } finally {
-      if (result._destroyAppInstance()) {
-        if (destroyAppInstanceTimer) {
-          clearTimeout(destroyAppInstanceTimer);
-        }
-      }
+      clearTimeout(destroyAppInstanceTimer);
     }
 
     return result;