This pull request addresses https://github.com/mozilla/sweet.js/issues/515. It enables conversion from the Shift AST format used in sweet.js to the Babel format using a fork of shift-spidermonkey-converter: https://github.com/gabejohnson/shift-spidermonkey-converter-js/tree/babel.
The conversion allows Sweet to avoid generating code and passing that code to Babel to be re-parsed.