Move recipe engine to third_party/recipe_engine.

third_party/recipe_engine/config.py

-# Copyright 2013 The Chromium Authors. All rights reserved.
+# Copyright 2013-2015 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.
 
 """Recipe Configuration Meta DSL.
 
 This module contains, essentially, a DSL for writing composable configurations.
 You start by defining a schema which describes how your configuration blobs will
 be structured, and what data they can contain. For example:
 
   FakeSchema = lambda main_val=True, mode='Happy': ConfigGroup(
@@ skipping 251 matching lines @@
       def default_config_vars():
         ret = {}
         for include in includes or []:
           item = self.CONFIG_ITEMS[include]
           ret.update(item.DEFAULT_CONFIG_VARS())
         if config_vars:
           ret.update(config_vars)
         return ret
       inner.DEFAULT_CONFIG_VARS = default_config_vars
 
-      assert name not in self.CONFIG_ITEMS
+      assert name not in self.CONFIG_ITEMS, (
+          '%s is already in CONFIG_ITEMS' % name)
       self.CONFIG_ITEMS[name] = inner
       if group:
         self.MUTEX_GROUPS.setdefault(group, set()).add(name)
       inner.IS_ROOT = is_root
       if is_root:
         assert not self.ROOT_CONFIG_ITEM, (
           'may only have one root config_ctx!')
         self.ROOT_CONFIG_ITEM = inner
         inner.IS_ROOT = True
       inner.NO_TEST = no_test or bool(deps)
@@ skipping 499 matching lines @@
     return self.data
 
   def reset(self):
     assert False
 
   def complete(self):
     return True
 
   def _is_default(self):
     return True