Make the externally connectable browser test clobber all of the builtins,

chrome/renderer/resources/extensions/entry_id_manager.js

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 var fileSystemNatives = requireNative('file_system_natives');
 
 var nameToIds = {};
 var idsToEntries = {};
 
 function computeName(entry) {
   return entry.filesystem.name + ':' + entry.fullPath;
 }
 
 function computeId(entry) {
   var fileSystemId = fileSystemNatives.CrackIsolatedFileSystemName(
       entry.filesystem.name);
   if (!fileSystemId)
     return null;
   // Strip the leading '/' from the path.
-  return fileSystemId + ':' + entry.fullPath.slice(1);
+  return fileSystemId + ':' + $String.slice(entry.fullPath, 1);
 }
 
 function registerEntry(id, entry) {
   var name = computeName(entry);
   nameToIds[name] = id;
   idsToEntries[id] = entry;
 }
 
 function getEntryId(entry) {
   var name = null;
@@ skipping 12 matching lines @@
   return id;
 }
 
 function getEntryById(id) {
   return idsToEntries[id];
 }
 
 exports.registerEntry = registerEntry;
 exports.getEntryId = getEntryId;
 exports.getEntryById = getEntryById;