Index: frontend/node_modules/mz/fs.js
===================================================================
--- frontend/node_modules/mz/fs.js	(revision 9af201e94b5a79beb92a30858fc54de4bc3b9449)
+++ frontend/node_modules/mz/fs.js	(revision 9af201e94b5a79beb92a30858fc54de4bc3b9449)
@@ -0,0 +1,62 @@
+
+var Promise = require('any-promise')
+var fs
+try {
+  fs = require('graceful-fs')
+} catch(err) {
+  fs = require('fs')
+}
+
+var api = [
+  'appendFile',
+  'chmod',
+  'chown',
+  'close',
+  'fchmod',
+  'fchown',
+  'fdatasync',
+  'fstat',
+  'fsync',
+  'ftruncate',
+  'futimes',
+  'lchown',
+  'link',
+  'lstat',
+  'mkdir',
+  'open',
+  'read',
+  'readFile',
+  'readdir',
+  'readlink',
+  'realpath',
+  'rename',
+  'rmdir',
+  'stat',
+  'symlink',
+  'truncate',
+  'unlink',
+  'utimes',
+  'write',
+  'writeFile'
+]
+
+typeof fs.access === 'function' && api.push('access')
+typeof fs.copyFile === 'function' && api.push('copyFile')
+typeof fs.mkdtemp === 'function' && api.push('mkdtemp')
+
+require('thenify-all').withCallback(fs, exports, api)
+
+exports.exists = function (filename, callback) {
+  // callback
+  if (typeof callback === 'function') {
+    return fs.stat(filename, function (err) {
+      callback(null, !err);
+    })
+  }
+  // or promise
+  return new Promise(function (resolve) {
+    fs.stat(filename, function (err) {
+      resolve(!err)
+    })
+  })
+}