fix(utils/decorator): handle desc return value

[buschtoens]

While writing tests for legacy decorators for rwjblue/sparkles-component#22, I noticed that the return value is ignored and that the descriptor needs to be mutated by reference.

I don't think that this is intentional, since we do the opposite for stage 2 decorators using the same callback function.

[stefanpenner]

This looks like it needs a regression test.

[buschtoens]

The build error seems to be unrelated. Can someone restart the build, please?

[buschtoens]

There's quite some auto-formatting noise, but since there is a .prettierrc.js, I just kept it, to get it over with. I can undo it though.

[buschtoens]

Which pattern do you prefer?

(NEEDS_STAGE_1_DECORATORS ? test : skip)('it ...', function(assert) { ... });

if (NEEDS_STAGE_1_DECORATORS) {
  test('it ...', function(assert) { ... });
}

[pzuraq]

Great job on this one!