Devserver: allow SubversionFileSystem to be pinned to a specific rev on construction

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

 # Copyright 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.
 
 from file_system import FileSystem, FileNotFoundError, StatInfo
 from future import Future
 
 class TestFileSystem(FileSystem):
   '''A FileSystem backed by an object. Create with an object representing file
   paths such that {'a': {'b': 'hello'}} will resolve Read('a/b') as 'hello',
@@ skipping 10 matching lines @@
     '''
     result = {}
     leaf = result
     for k in base.split('/'):
       leaf[k] = {}
       leaf = leaf[k]
     leaf.update(obj)
     return result
 
   def __init__(self, obj):
+    assert obj is not None
     self._obj = obj
     self._path_stats = {}
     self._global_stat = 0
 
   #
   # FileSystem implementation.
   #
 
   def Read(self, paths, binary=False):
     test_fs = self
@@ skipping 66 matching lines @@
   #
 
   def IncrementStat(self, path=None):
     if path is not None:
       self._path_stats[path] = self._path_stats.get(path, 0) + 1
     else:
       self._global_stat += 1
 
   def GetIdentity(self):
     return self.__class__.__name__