Fix issue with generation of 'null' value enum fields

[jcollins-g]

Fixes #1445. Adding Paul to the reviewers as a comment in ConstFieldElementImpl suggested he might have context as to whether the change to how we compute enum indexes is actually an improvement over some variant of the original hack.

[jcollins-g]

AppVeyor builds are pub errors and I don't have access to retry them. @devoncarew?

[stereotype441]

This definitely works but it took a bit of thinking before I understood why. Essentially what's happening here is that the fields in an enum class E are all static fields having type E, except for the two "special" fields index (a non-static field having type int) and values (a static field having type List<E>). You only want your new behavior for the non-special fields, and it works because only the type E has a non-static member index whose value is an int; the types int and List<E> don't.

If I'm understanding the code correctly there's a getter FieldElement.isEnumConstant that does what you want. So I think you could replace lines 2140-2161 with something like:

if (e.isEnumConstant) {
  int index = e.computeConstantValue().getField('index').toIntValue();
  newModelElement = new EnumField.forConstant(index, e, library, getter);
} else if (e.enclosingElement.isEnum) {
  newModelElement = new EnumField(e, library, getter, setter);
} else {
  newModelElement = new Field(e, library, getter, setter);
}

[jcollins-g]

Oh, that's so much better. This logic is what I intended: if I had seen/thought to look for e.isEnumConstant I would have written it exactly like this. Thanks!

[stereotype441]

I made a comment about how enum values are computed. I am not too familiar with DartDoc so I'll leave the full review to Keerti.

[keertip]

why was this removed?

[jcollins-g]

It was a duplicate, it was already addAll on line 454.

[keertip]

that line is not shown in the review :)