Migrate from ConstVariableElement to ConstVariableFragment

[fshcheglov]

https://dart-review.googlesource.com/c/sdk/+/434742

I'm making name changes to make sure all fragment implementation classes are referred to as fragments. However, dartdoc references one non-api class, so it would be good to switch to the new name, or stop using it altogether if analyzer provides the necessary API.

[srawlins]

Which class is that? I see zero code currently with 'ElementImpl':

$ git grep ElementImpl
CHANGELOG.md:* Add better support for GenericFunctionTypeElementImpl (#1506, #1509)
CHANGELOG.md:* Add support for GenericFunctionTypeElementImpl (#1495)

[srawlins]

Oops, I see it is in the title, haha. My apologies.

We cannot do any migration from ConstVariableElement until analyzer 8.0.0 is released, correct?

[sigurdm]

@scheglov do you have the context here?