Loading routes do not work in FastBoot

I get the following error in my app:

```
There was an error running your app in fastboot. More info about the error:
Error: a glimmer transaction was begun, but one already exists. You may have a nested transaction
at debugAssert (.../tmp/broccoli_merge_trees-output_path-Y53V5i2J.tmp/assets/@glimmer/util.js:62:1)
at Environment.begin (.../tmp/broccoli_merge_trees-output_path-Y53V5i2J.tmp/assets/@glimmer/runtime.js:6378:1)
at Environment.begin (.../tmp/broccoli_merge_trees-output_path-Y53V5i2J.tmp/assets/ember-glimmer/environment.js:273:1)
at InertRenderer._renderRoots (.../tmp/broccoli_merge_trees-output_path-Y53V5i2J.tmp/assets/ember-glimmer/renderer.js:311:1)
at InertRenderer._renderRootsTransaction (.../tmp/broccoli_merge_trees-output_path-Y53V5i2J.tmp/assets/ember-glimmer/renderer.js:379:1)
at InertRenderer._revalidate (.../tmp/broccoli_merge_trees-output_path-Y53V5i2J.tmp/assets/ember-glimmer/renderer.js:418:1)
at invokeWithOnError (.../tmp/broccoli_merge_trees-output_path-Y53V5i2J.tmp/assets/backburner.js:281:1)
at Queue.flush (.../tmp/broccoli_merge_trees-output_path-Y53V5i2J.tmp/assets/backburner.js:152:1)
at DeferredActionQueues.flush (.../tmp/broccoli_merge_trees-output_path-Y53V5i2J.tmp/assets/backburner.js:343:1)
at Backburner.end (.../tmp/broccoli_merge_trees-output_path-Y53V5i2J.tmp/assets/backburner.js:451:1)
at Timeout.Backburner._boundAutorunEnd [as _onTimeout] (.../tmp/broccoli_merge_trees-output_path-Y53V5i2J.tmp/assets/backburner.js:413:1)
at ontimeout (timers.js:469:11)
at tryOnTimeout (timers.js:304:5)
at Timer.listOnTimeout (timers.js:264:5)
```

At first I thought liquid-fire ([#530](https://github.com/ember-animation/liquid-fire/issues/530)) was causing this. But after removing liquid-fire I got the error above.