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

chrome/common/extensions/docs/server2/fake_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 os
 import posixpath
 
 from future import Future
 from path_util import AssertIsDirectory, IsDirectory
 
@@ skipping 111 matching lines @@
   def __init__(self, fetcher):
     self._fetcher = fetcher
     self.Reset()
 
   def Fetch(self, url, **kwargs):
     self._fetch_count += 1
     return self._fetcher.Fetch(url, **kwargs)
 
   def FetchAsync(self, url, **kwargs):
     self._fetch_async_count += 1
-    future = self._fetcher.FetchAsync(url, **kwargs)
-    def resolve():
+    def next(result):
       self._fetch_resolve_count += 1
-      return future.Get()
-    return Future(callback=resolve)
+      return result
+    return self._fetcher.FetchAsync(url, **kwargs).Then(next)
 
   def CheckAndReset(self,
                     fetch_count=0,
                     fetch_async_count=0,
                     fetch_resolve_count=0):
     errors = []
     for desc, expected, actual in (
         ('fetch_count', fetch_count, self._fetch_count),
         ('fetch_async_count', fetch_async_count, self._fetch_async_count),
         ('fetch_resolve_count', fetch_resolve_count,
                                 self._fetch_resolve_count)):
       if actual != expected:
         errors.append('%s: expected %s got %s' % (desc, expected, actual))
     try:
       return (len(errors) == 0, ', '.join(errors))
     finally:
       self.Reset()
 
   def Reset(self):
     self._fetch_count = 0
     self._fetch_async_count = 0
     self._fetch_resolve_count = 0