Pass Babel AST to Prettier rather than re-parsing

[bvaughn]

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