level-filesystem fails to bundle [was: Bundle fail]

error:

```
Bundling error:

---FLAGRANT SYSTEM ERROR---

--- error #0: ---

Error: "browserify exited with code 8"

code: 8
stderr:
/home/admin/browserify-cdn/node_modules/browserify/index.js:628
var keys = Object.keys(a);
^
RangeError: Maximum call stack size exceeded

dirPath: /tmp/level-filesystem11465-1072-wx7l81
```

code:

``` js
var curry = require('ap')
var async = require('async')
var leveljs = require('level-js')
var levelup = require('levelup')
var filesystem = require('level-filesystem')
var treeify = require('treeify').asTree

var db = levelup('foo', { db: leveljs })
var fs = filesystem(db)
window.fs = fs

seedFs(function(){

buildIndex(function(error, fsObj) {
console.log( treeify(fsObj, false) )
})

})

function seedFs(callback) {

async.series([
curry(['index.js','DATA'], fs.writeFile),
curry(['package.json','DATA'], fs.writeFile),
curry(['lib'], fs.mkdir),
curry(['lib/util.js','DATA'], fs.writeFile),
], callback)

}

function buildIndex(callback) {
var rootPath = '/'
var fsObj = new Directory()
var rootDirObj = new Directory()
fsObj[rootPath] = rootDirObj
updateDir({dirPath: rootPath, dirObj: rootDirObj }, callback)
}

function updateDir(opts, callback) {
var dirPath = opts.dirPath
var dirObj = opts.dirObj

fs.readdir(dirPath, function(error, entityNameList) {

var entityPathList = entityNameList.map(function(entityName) {
return dirPath + '/' +entityName
})

async.map(entityPathList, fs.stat, function(err, statList) {

// add files + dirs to directoryObject,
// filter out subdirs for recursive updates
var subDirs = []
statList.map(function(stats, index) {

var entryName = entityNameList[index]
var entryPath = dirPath + '/'+ entryName

switch (stats.type) {
case 'file':
// dirObj[entryName] = stats
dirObj[entryName] = true
break

case 'directory':
var newDir = new Directory()
dirObj[entryName] = newDir
subDirs.push({
dirPath: entryPath,
dirObj: newDir,
})
break

default:
throw new Error('unknown fs entity type')
break
}

})

// recursively update subdirs
async.parallel(subDirs, updateDir, function() {
callback(null, dirObj)
})

})

})
}

function Directory(opts) {}
```

Start by running the JavaScript reproduction from the issue and confirm the failure at browserify/index.js:628, using the listed level-filesystem setup. Trace the bundling path around the reported maximum call stack error; done means this example bundles successfully without the recursive failure.

Written by the indexing model from the issue text.