Upgrading to koa 2.2.0 gives unexpected token { in koa/lib/response.js
After making some changes to my development environment, I was having issues getting koa 2.0 to work. It would complain that it can't find the function '_regeneratorRuntime' (related to the generators I suppose), ie:
```
Users/rw3iss/Sites/testapp_koa/src/server/routes/users/index.js:213
var _marked = [search, saveUser].map(_regeneratorRuntime.mark);
                                     ^
ReferenceError: _regeneratorRuntime is not defined
```
(I have "syntax-async-functions" and "transform-regenerator" set in my .babelrc plugins entry)

It used to work before. I then upgraded to Koa 2.2.0, and node v7, and now when I run the app, it seems it can't interpret some es6 stuff, ie:

```
/Users/rw3iss/Sites/testapp_koa/src/node_modules/koa/lib/response.js:47
    const { res } = this;
          ^
```

My development environment for the main app all works and compiles (with babel-node) using this and other es6 syntax. Not sure why it's complaining for underlying npm libraries. 
Anyone know how to get around this?


