Make output placeholders like json.output index-able by name.

recipe_modules/json/test_api.py

 # Copyright 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.
 
 import json
 
 from recipe_engine import recipe_test_api
 
 class JsonTestApi(recipe_test_api.RecipeTestApi):
   @recipe_test_api.placeholder_step_data
   @staticmethod
-  def output(data, retcode=None):
-    return json.dumps(data), retcode
+  def output(data, retcode=None, name=None):
+    return json.dumps(data), retcode, name
 
-  def output_stream(self, data, stream='stdout', retcode=None):
+  def output_stream(self, data, stream='stdout', retcode=None, name=None):
     assert stream in ('stdout', 'stderr')
     ret = recipe_test_api.StepTestData()
-    setattr(ret, stream, self.output(data, retcode).unwrap_output_placeholder())
+    step_data = self.output(data, retcode=retcode, name=name)
+    setattr(ret, stream, step_data.unwrap_output_placeholder())
     return ret