I'd also add that Typescript would be a very bad language choice anyway. If we're going to have a backward-incompatible change, we should do things the right way (after all, we're stuck with it forever).

Don't make a type system work around the really bad parts of JS. Instead, don't allow them in typed modules. Don't settle for an intentionally unsound (aka broken ) type system. Add an actually sound (probably hindley-milner based) type system that provides usable type info to the JIT so we can guarantee fast, safe code.