[package:js] draft proposal to improve JS interop, see #35084

Change-Id: I77a01469113cdfe6c9c3533cfb398ab97d5cd3b0
Reviewed-on: https://dart-review.googlesource.com/c/89404
Reviewed-by: Vijay Menon <[email protected]>
Commit-Queue: Jenny Messerly <[email protected]>

Author: Jenny Messerly