package:js and dart:js_util is not restrictive enough for our purposes and may lead to unexpected behavior (either due to odd semantics or lack of support) and/or performance cliffs. This will require migrating internal usage, but for now we should treat these two libraries as deprecated for dart2wasm. Support for these libraries will continue on dart2js and ddc until we feel ready to start deprecating and migrating.

This will require migrating any Flutter usages to use dart:js_interop, but we can allowlist existing usages for now and disallow these libraries in dart2wasm for 3.3.

This will also require making sure dart:js_interop reaches parity with package:js and dart:js_util by providing any missing functionalities.

cc @kevmoo