Fix imports for ember 3.5

[gossi]

The recent way of importing the installers stopped working, when we upgraded our app to ember 3.5. This fix lazy imports the installer and provide the required functionality for our tests.

That said, I haven't tests this patch against lower ember versions.

[workmanw]

@gossi Can you check to see if this issue is still necessary after #27 ? It seems to related, but not entirely. If so, can you please update the index.js?

[gossi]

I remember this was a hotfix after the 3.5 release of ember. We are on 0.2.1, which is working for us (maybe we did some other black magic :D @buschtoens ?).

@Turbo87 helped me to get this going. I trust him to say how this one harmonizes with his PR and whether this is needed or can be closed.

[Turbo87]

#27 was just a quickfix to get ember-cli/ember-resolver#314 running again, but I think the solution here might be better in general. Ideally, we could indeed concat everything in the right order using Broccoli, but with the current hooks, that's more complicated than this proposed solution.

[workmanw]

The recent way of importing the installers stopped working...

I read that as this addon no longer works at all with ember 3.5 and that's why I was trying to determine if @Turbo87's fix also resolved this issue.

[stefanpenner]

I am upstreaming these assertions into ember-qunit

I will review this PR, and try and incorporate appropriate changes upstream.

When all this lands, my plan is to deprecate this library entirely in-favor of it's assertions simply being officially supported: #29