`super` in concise methods not working

Using `super` in method shorthands ("concise methods") is valid syntax, but Buble doesn't think so. See [the example](https://buble.surge.sh/#%20%20%20%20const%20B%20%3D%20A.subclass(%7B%0A%20%20%20%20%20%20%20%20foo(n)%20%7B%0A%20%20%20%20%20%20%20%20%20%20%20%20if%20(n%20%3E%20100)%20return%20-1%3B%0A%20%20%20%20%20%20%20%20%20%20%20%20return%20super.foo(n%2B1)%3B%20%2F%2F%20RIGHT%20HERE%0A%20%20%20%20%20%20%20%20%7D%0A%20%20%20%20%7D)).

For reference, here's [Babel's example](http://babeljs.io/repl/#?babili=false&browsers=ie%208&build=&builtIns=false&code_lz=AQ4YwewOwZwF2AIWAXmAQQHQwK4CMwAbAQxhgAoBvAKFDuADMIJyoBKYG-74ASweCtgAPmABGAAwSOAJwCmcHDKjAAtGIDctHnXmLlwXAAc5MzExZQA1GLYbgAegfAASgEkA4gAkAKsC8Aoi4B2twAvqHAYWxAA&debug=false&forceAllTransforms=false&shippedProposals=false&circleciRepo=&evaluate=true&fileSize=false&lineWrap=true&presets=es2015%2Ces2016%2Ces2017%2Creact%2Cstage-2%2Ces2015-loose%2Cenv&prettier=false&targets=&version=6.26.0&envVersion=1.6.2).

In earlier versions of Buble, the [error is not helpful at all](https://github.com/rollup/rollup/issues/1623#issuecomment-378833862), resulting in

```
Cannot read property 'magicString' of undefined
```

I had to hunt the line down by deleting code until I was left with the culprit.

Note, I'm running only Buble (no Rollup).