[context] Add path prefix/suffix manipulation.

recipe_modules/context/tests/env.py

Index: recipe_modules/context/tests/env.py
diff --git a/recipe_modules/context/tests/env.py b/recipe_modules/context/tests/env.py
index 817050a199d4f9ed001d5b2e33e1cedd08fd14e1..867e27ea9f90e8c30a6c6a1c34c46a2526bbab5b 100644
--- a/recipe_modules/context/tests/env.py
+++ b/recipe_modules/context/tests/env.py
@@ -18,6 +18,24 @@ def RunSteps(api):
     with api.context(env={"SOMETHING_ELSE": "0"}):
       api.step('with 2 envs', ['echo', 'hello'])
 
+  with api.context(env={'FOO': 'bar'}):
+    api.step('env step', ['bash', '-c', 'echo $FOO'])
+
+    base_path = 'foo%s%%(FOO)s%sbaz' % (api.path.pathsep, api.path.pathsep)
+    with api.context(env={'FOO': base_path}):
+      api.step('env step augmented', ['bash', '-c', 'echo $FOO'])
+
+      pants = api.path['start_dir'].join('pants')
+      shirt = api.path['start_dir'].join('shirt')
+      with api.context(env={'FOO': api.context.Prefix(pants, shirt)}):
+        api.step('env step with prefix', ['bash', '-c', 'echo $FOO'])
+
+  # Can set the path of default environment variables.
+  with api.context(env={'FOO': api.context.Prefix(shirt)}):
+    api.step('env with default value',
+             ['bash', '-c', 'echo $FOO'])
+
+
 def GenTests(api):
   yield api.test('basic')