[dart fix] rename change not suggested for casts

[devoncarew]

When contributing some class rename dart fix fixes for package:web, a rename is not offered for as casts:

import 'package:web/web.dart';

void main() {
  final HtmlDocument foo = document; // Rename (from HtmlDocument => Document) _is_ offered here

  final doc = document as HtmlDocument; // Rename not offered here (but should be)

  final div = doc.querySelector('div')!;
  div.text = 'Text set at ${DateTime.now()}';
}

More context in dart-lang/web#141.

cc @keertip