Avoid implicit injection _fastbootInfo deprecation pt 2

Author: snewcomer

packages/ember-cli-fastboot/addon/services/fastboot.js

@@ -89,9 +89,19 @@ const FastBootService = Service.extend({
     return RequestObject.create({ request: get(this, '_fastbootInfo.request') });
   }),
 
-  _fastbootInfo: computed(function() {
-    // this getter is to avoid deprecation from [RFC - 680](https://github.com/emberjs/rfcs/pull/680)
-    return getOwner(this).lookup('info:-fastboot');
+  _fastbootInfo: computed({
+    get() {
+      if (this.__fastbootInfo) {
+        return this.__fastbootInfo;
+      }
+
+      // this getter is to avoid deprecation from [RFC - 680](https://github.com/emberjs/rfcs/pull/680)
+      return getOwner(this).lookup('info:-fastboot');
+    },
+    set(_key, value) {
+      this.__fastbootInfo = value;
+      return value;
+    }
   }),
 
   deferRendering(promise) {