Revert of Track step nesting in StreamEngine.

recipe_modules/step/config.py

 # Copyright 2013 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 import collections
 
+from recipe_engine.config import List
 from recipe_engine.config import (config_item_context, ConfigGroup, ConfigList,
-                                  Dict, Single, Set, List)
+                          Dict, Single, Set)
 from recipe_engine.config_types import Path
 from recipe_engine.util import Placeholder
 
 
 def BaseConfig(**_kwargs):
   def render_cmd(lst):
     return [(x if isinstance(x, Placeholder) else str(x)) for x in lst]
 
   return ConfigGroup(
     # For compatibility with buildbot, the step name must be ascii, which is why
@@ skipping 31 matching lines @@
     step_nest_level = Single(int, required=False),
   )
 
 
 config_ctx = config_item_context(BaseConfig)
 
 @config_ctx()
 def test(c):  # pragma: no cover
   c.name = 'test'
   c.cmd = [Path('[CHECKOUT]', 'build', 'tools', 'cool_script.py')]