Track step nesting in StreamEngine.

recipe_engine/run.py

Index: recipe_engine/run.py
diff --git a/recipe_engine/run.py b/recipe_engine/run.py
index 17b8890053c65d2ba19bda1dc8ce5adac8914e05..385050d05148c2d3246767e64c15700e023db69a 100644
--- a/recipe_engine/run.py
+++ b/recipe_engine/run.py
@@ -296,8 +296,8 @@ class RecipeEngine(object):
     * step - uses engine.create_step(...), and previous_step_result.
   """
 
-  ActiveStep = collections.namedtuple('ActiveStep',
-                                      'step step_result open_step nest_level')
+  ActiveStep = collections.namedtuple('ActiveStep', (
+      'step', 'step_result', 'open_step', 'nest_level'))
 
   def __init__(self, step_runner, properties, universe_view):
     """See run_steps() for parameter meanings."""
@@ -368,7 +368,7 @@ class RecipeEngine(object):
 
       step_result = None
 
-      nest_level = step.pop('step_nest_level', 0)
+      nest_level = step.get('step_nest_level', 0)
       self._close_to_level(nest_level)
 
       open_step = self._step_runner.open_step(step)
@@ -377,8 +377,6 @@ class RecipeEngine(object):
           step_result=None,
           open_step=open_step,
           nest_level=nest_level))
-      if nest_level:
-        open_step.stream.set_nest_level(nest_level)
 
       step_result = open_step.run()
       self._step_stack[-1] = (