Pass Babel AST to Prettier rather than re-parsing

See _TODO_ comments in `js/repl/src/compile`.

It would be more efficient to pass Prettier the AST Babel has already created. However we can't do this until we upgrade to Babel 7 (since that's what Prettier uses).

It also seems suboptimal that Prettier only exposes this functionality behind a `__debug` property (`prettier.__debug.formatAST`). Maybe we could reach out to the Prettier team about a more formal API for this?

This is a follow-up item for PR #1297