Error combining with 6to5ify

I'm trying to do a brfs transform after 6to5ify, but get the following error.

```
browserify()
.require("main", { entry: true })
.transform(6to5ify)
.transform(brfs)
.bundle()
.pipe(fs.createWriteStream(path.join('app.js')));
```

```
events.js:72
throw er; // Unhandled 'error' event
^
Error: tried to statically call { readFile: [Function: readFile], readFileSync: [Function: readFileSync], readdir: [Function: readdir], readdirSync: [Function: readdirSync] } as a function while parsing file: /Users/akula/foo/main.js while parsing file: /Users/akula/foo/main.js
at error (/Users/akula/foo/node_modules/brfs/node_modules/static-module/index.js:71:49)
at traverse (/Users/akula/foo/node_modules/brfs/node_modules/static-module/index.js:247:24)
at walk (/Users/akula/foo/node_modules/brfs/node_modules/static-module/index.js:216:13)
at walk (/Users/akula/foo/node_modules/brfs/node_modules/static-module/node_modules/falafel/index.js:60:9)
at /Users/akula/foo/node_modules/brfs/node_modules/static-module/node_modules/falafel/index.js:51:25
at Array.forEach (native)
at forEach (/Users/akula/foo/node_modules/brfs/node_modules/static-module/node_modules/falafel/index.js:8:31)
at /Users/akula/foo/node_modules/brfs/node_modules/static-module/node_modules/falafel/index.js:49:17
at Array.forEach (native)
at forEach (/Users/akula/foo/node_modules/brfs/node_modules/static-module/node_modules/falafel/index.js:8:31)
```

btw, there's a double "while parsing file" in the message.

My main.js looks like this.

```
import fs from 'fs';

var foo = fs.readFileSync(__dirname + '/foo.html');
console.log(foo.toString());
```

If I change the import to a require and don't do 6to5ify it works fine, but together it gives the error.