Add support for LUCI_CONTEXT.

recipe_modules/context/examples/full.py

 # Copyright 2017 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.
 
 from recipe_engine import recipe_api, config
 
 DEPS = [
   'context',
   'path',
   'raw_io',
@@ skipping 48 matching lines @@
 
   # can increment nest level... note that this is a low level api, prefer
   # api.step.nest instead:
   # YES:
   with api.step.nest('nested'):
     api.step('properly indented', ['bash', '-c', 'echo yay!'])
   # AVOID:
   with api.context(increment_nest_level=True):
     api.step('indented with wrong name', ['bash', '-c', 'echo indent?'])
 
+  # Can read initial LUCI_CONTEXT value.
+  val = api.context.luci_context['INITIAL']['k']
+  api.step('value in the context', ['echo', val])
+
+  # Can modify it.
+  with api.context(luci_context={'INITIAL': None, 'OTHER': {'k': 'zzz'}}):
+    val = api.context.luci_context['OTHER']['k']
+    api.step('value in the context', ['echo', val])
+
 
 def GenTests(api):
-  yield api.test('basic')
+  yield api.test('basic') + api.context.luci_context({'INITIAL': {'k': 'val'}})