Add simple depends_on API.

recipes/engine_tests/depend_on/bottom.py

+from recipe_engine.recipe_api import Property
+from recipe_engine import config
+
+DEPS = [
+    'raw_io',
+    'recipe_engine/properties',
+    'step',
+]
+
+PROPERTIES = {
+    'number': Property(kind=int),
+}
+
+RETURN_SCHEMA = config.ReturnSchema(
+    number_times_two=config.Single(int),
+)
+
+def RunSteps(api, number):
+  # Newline cause bc complains if it doesn't have it
+  num_s = '%s*%s\n' % (number, 2)
+  result = api.step(
+      'calc it', ['bc'],
+      stdin=api.raw_io.input(data=num_s),
+      stdout=api.raw_io.output('out'))
+  return RETURN_SCHEMA(number_times_two=int(result.stdout))
+
+def GenTests(api):
+  yield (
+      api.test('basic') +
+      api.properties(number=3) +
+      api.step_data('calc it',
+                    stdout=api.raw_io.output('6'))
+  )