Revert of Track step nesting in StreamEngine.

recipe_engine/unittests/stream_test.py

 #!/usr/bin/env python
 # Copyright 2015 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 cStringIO
 import os
 import sys
 import unittest
 
@@ skipping 18 matching lines @@
 
     foo.add_step_link('read it online!', 'https://foospoemtobar.com/')
     foo.add_step_summary_text('read a killer poem and took a bow')
     foo.trigger('{"builderName":["bar\'s fantasies"]}')
     foo.close()
 
     # TODO(luqui): N.B. stream interleaving is not really possible with
     # subannotations, since the subannotator stream could have changed the
     # active step.  To do this right we would need to parse and re-emit
     # subannotations.
-    bars_baby = engine.new_step_stream(
-        'bar\'s baby',
-        allow_subannotations=True,
-        nest_level=1)
+    bars_baby = engine.new_step_stream('bar\'s baby', allow_subannotations=True)
+    bars_baby.set_nest_level(1)
     bars_baby.write_line('I\'m in bar\'s imagination!!')
     bars_baby.write_line('@@@STEP_WARNINGS@@@')
     bars_baby.reset_subannotation_state()
     bars_baby.close()
 
     bar.set_build_property('is_babycrazy', 'true')
     bar.write_line('bar tries to kiss foo, but foo already left')
     bar.write_line('@@@KISS@foo@@@')
     bar.set_step_status('EXCEPTION')
     bar.close()
@@ skipping 106 matching lines @@
 
   def test_buildbot_status_constraint(self):
     with stream.StreamEngineInvariants() as engine:
       foo = engine.new_step_stream('foo')
       foo.set_step_status('FAILURE')
       with self.assertRaises(AssertionError):
         foo.set_step_status('SUCCESS')
 
 if __name__ == '__main__':
   unittest.main()