This only addresses the following:
- Removes `JSVoid` in favor of `void`. `JSVoid`, which is just a
typedef, should be deprecated and removed eventually.
- Removes extra interop methods that were needed to make sure arity was
correct. These are no longer needed as interop methods are
invocation-sensitive and calls to them do not forward more arguments
than were passed by the user.
- Removes unnecessary annotations on extensions.
Still left to do (not comprehensive):
- Migrate to extension types.
- Remove forwarding methods that are only used to convert primitives in
favor of changing the interop method to include the Dart primitive type.
It might be a bit slower in dart2wasm to do this, but will need to
verify and fix it if that's the case.
- Remove usage of `js_util`.
- See if we can replace types with their `package:web` equivalent for
easier maintenance.
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