Tracked properties are non-enumerable

Class properties are enumerable:

```js
class Foo {
someProp;
}
console.log(Object.keys(new Foo()));
// prints ["someProp"]
```

But if we mark one as `@tracked`, it becomes non-enumerable:

```js
class Foo {
@tracked
someProp;
}
console.log(Object.keys(new Foo()));
// prints []
```

This seems bad and confusing so I suspect it is unintentional.