Add simple depends_on API.

recipe_engine/recipe_api.py

Index: recipe_engine/recipe_api.py
diff --git a/recipe_engine/recipe_api.py b/recipe_engine/recipe_api.py
index 985abe200e90516e993c956a9840d6c695a39d8a..ce4cbe34bc27b63a6e4b553387c31fe6526ee3ce 100644
--- a/recipe_engine/recipe_api.py
+++ b/recipe_engine/recipe_api.py
@@ -4,8 +4,12 @@
 
 from __future__ import absolute_import
 import contextlib
+import json
 import keyword
+import os
 import re
+import subprocess
+import tempfile
 import types
 
 from functools import wraps
@@ -454,8 +458,11 @@ class RecipeApi(RecipeApiPlain):
 
 
 class RecipeScriptApi(RecipeApiPlain, ModuleInjectionSite):
-  pass
+  def depend_on(self, recipe, properties, **kwargs):
+    return self._engine.depend_on(recipe, properties, **kwargs)
 
+  def depend_on_test(self, recipe, properties, value, **kwargs):
+    return self._engine.depend_on_test(recipe, properties, value, **kwargs)
 
 # This is a sentinel object for the Property system. This allows users to
 # specify a default of None that will actually be respected.