[build] Remove dependency on ES stage-0 and stage-1 proposals

This removes the dependencies on stage0 and stage1 proposals for ECMAScript, which have a significant risk of not being integrated (see the :: auto-binding proposal that has never made it to stage 2 in two years+). It removes them, as an attempt to simplify testing other languages/type system annotations on the server, and to make code cleaner.

I'd have wanted to use stage-3 even, but the class properties proposal (extensively used in the client) is actually stage 2. Oh, well.