Incorrect result with async arrow functions when not using `babel-preset-es2015`

I tried kneden out on some test files where I was previously using babel-transform-async-to-generator and came across a failing case.

In this project I wasn't using babel-preset-es2015 as I just wanted to compile async/await down to es6.

I've simplified the test case down to this:

.babelrc

{
  "plugins": ["../../../src"]
}

actual.js

function test() {
  let server;
  before(async () => server = await TestServer.start());
}

The output I get from running the test is:

function test() {
  let server;
  before(() => {
    return Promise.resolve();
  });
}

which is definitely not right :)

If I add "presets": ["es2015"] to .babelrc in the fixture dir I get the more correct looking:

function test() {
  var server = undefined;
  before(function () {
    return Promise.resolve().then(function () {
      return TestServer.start();
    }).then(function (_resp) {
      return server = _resp;
    });
  });
}