Ensure that destroyAppInstanceInMs does not throw if completed

Author: Robert Jackson

src/ember-app.js

@@ -187,7 +187,10 @@ class EmberApp {
   /**
    * Perform any cleanup that is needed
    */
-  destroy() {}
+  destroy() {
+    // TODO: expose as public api (through the top level) so that we can
+    // cleanup pre-warmed visits
+  }
 
   /**
    * @private

src/result.js

@@ -152,7 +152,7 @@ class Result {
 
   _destroy() {
     if (this.isDestroyed) {
-      return true;
+      return false;
     }
 
     this.isDestroyed = true;
@@ -164,6 +164,8 @@ class Result {
     if (this.applicationInstance !== undefined) {
       this.applicationInstance.destroy();
     }
+
+    return true;
   }
 
   _finalizeHTML() {