This version is needed so that dart:js_interop can move to extension
types. Also adds some code to handle some breaking changes:

- Body -> Response. Body was an IDL interface mixin type we exposed in
dart:html. Going forward, users should either use Request or Response.
- Casts to JSAny. These are temporary until we move package:web types to
extension types. Currently, package:web types can't implement JSObject
as JSObject will move to be an extension type itself.
Co-authored-by: Kevin Moore <kevmoo@users.noreply.github.com>