Docserver: Add FileSystem.Exists method and start sanity checking path format

chrome/common/extensions/docs/server2/appengine_url_fetcher.py

 # Copyright (c) 2012 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.
 
 import base64
 import posixpath
 
 from appengine_wrappers import GetAppVersion, urlfetch
 from future import Future
 
@@ skipping 15 matching lines @@
     headers['Authorization'] = 'Basic %s' % base64.b64encode(
         '%s:%s' % (username, password))
   return headers
 
 
 class AppEngineUrlFetcher(object):
   """A wrapper around the App Engine urlfetch module that allows for easy
   async fetches.
   """
   def __init__(self, base_path=None):
-    assert base_path is None or not base_path.endswith('/')
+    assert base_path is None or not base_path.endswith('/'), base_path
     self._base_path = base_path
 
   def Fetch(self, url, username=None, password=None):
     """Fetches a file synchronously.
     """
     return urlfetch.fetch(self._FromBasePath(url),
                           headers=_MakeHeaders(username, password))
 
   def FetchAsync(self, url, username=None, password=None):
     """Fetches a file asynchronously, and returns a Future with the result.
     """
     rpc = urlfetch.create_rpc()
     urlfetch.make_fetch_call(rpc,
                              self._FromBasePath(url),
                              headers=_MakeHeaders(username, password))
     return Future(delegate=_AsyncFetchDelegate(rpc))
 
   def _FromBasePath(self, url):
-    assert not url.startswith('/')
+    assert not url.startswith('/'), url
     if self._base_path is not None:
       url = posixpath.join(self._base_path, url) if url else self._base_path
     return url