The primary changes are:

• Make the rpc<R> function require the fn argument.
• Avoid decoding and parsing the request body multiple times in rpc<R> when there's a RequestException
• Instead of Element.div(), Element.p(), and similar, use the type factory constructors (DivElement()) for easier migration to package:web.
• Use e.getAttribute('attribute-key') instead of e.attributes['attribute-key'] as it avoids creating an unnecessary map but also makes migration to package:web easier.
• Avoid a few non-null checks and assertions with slight restructurings