Docserver: Update Future.Then() to be more Promise-like

chrome/common/extensions/docs/server2/mock_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.
 
 import posixpath
 
 from file_system import FileSystem, FileNotFoundError
 from future import Future
 from test_file_system import _List, _StatTracker, TestFileSystem
 from path_util import IsDirectory
@@ skipping 24 matching lines @@
 
   #
   # FileSystem implementation.
   #
 
   def Read(self, paths, skip_not_found=False):
     '''Reads |paths| from |_file_system|, then applies the most recent update
     from |_updates|, if any.
     '''
     self._read_count += 1
-    future_result = self._file_system.Read(paths, skip_not_found=skip_not_found)
-    def resolve():
+    def next(result):
       self._read_resolve_count += 1
-      result = future_result.Get()
       for path in result.iterkeys():
         update = self._GetMostRecentUpdate(path)
         if update is not None:
           result[path] = update
       return result
-    return Future(callback=resolve)
+    return self._file_system.Read(paths,
+                                  skip_not_found=skip_not_found).Then(next)
 
   def Refresh(self):
     return self._file_system.Refresh()
 
   def _GetMostRecentUpdate(self, path):
     '''Returns the latest update for the file at |path|, or None if |path|
     has never been updated.
     '''
     for update in reversed(self._updates):
       try:
@@ skipping 59 matching lines @@
     self._read_resolve_count = 0
     self._stat_count = 0
 
   def Update(self, update):
     self._updates.append(TestFileSystem(update))
     for path in _List(update).iterkeys():
       # Any files (not directories) which changed are now at the version
       # derived from |_updates|.
       if not IsDirectory(path):
         self._stat_tracker.SetVersion(path, len(self._updates))